There isn't 'no use' for partial and appendage cyberlimbs. You forget the main use of partial parts (as they were intended, at that). It's meant to replace the body-parts you lose in a fight or something. Your mage got his hand cut off; is he going to want a full limb put on, or just a replacement hand to lessen the Essence loss? Your cheapskate poor guy isn't going to want to shell out triple the price before even getting any upgrades for his stuff. It's also good if you only need that much Capacity. You seem to be looking at this from a strict meta-perspective only, which still isn't all that valid. Just because there's a bigger gun, doesn't mean you need it.
Indeed, I am looking at it from a strict meta-perspective. Because you don't break rules when you take a Cyberarm to replace a lost arm, you break rules if you push the limits intentionally from the get-go. If you managed to lose a hand, sure, get a replacement. You're main reason for a replacement will likely be to replace your hand, not to metagame and try and create a ridiculous combination of stats. The main reason for a cyberlimb is functionality to me, that's the point of Enhanced Strength and Enhanced Agility and Customized of each. If I was an average Joe who got his arm cut off in an accident, I would buy a basic off the shelf cyberlimb, possibly get it Alphaware, but it wouldn't have hidden weapons, it would have super tricked out Agility and Strength, it would just be a replacement. I doubt a GM would get upset over taking a basic limb with no special benefits that could possibly break the system.
Regardless, the point of the questions is do you consider something like this munchkining and trying to cheat the system? If you aren't trying to super optimize, I doubt it will be a problem.
There's also no indication that you can stack lower limb and hand/foot cyberlimbs. Your idea implies you can replace your entire forearm but still have a meat hand flopping around at the end, for one; secondly, your hands and feet are generally counted as a part of the lower limb. Cyber-hands and feet are also included with partial limbs in what tests the augmentations can apply to. While it's irritatingly unclear, common sense should apply; you can't take a full cyberlimb AND a partial cyberlimb on the same arm, and a full cyberlimb does include the hand.
I have no idea if hands/feet an lower limbs stack. I thought a lower arm was literally your Elbow to your Wrist, but if it includes your hand, then they don't stack. I can admit being wrong there.
Your point that Armor mods can fit into a cyber hand/foot is valid; Capacity 4 could fit all three levels of Armor in each. I'd personally just apply this paragraph to anyone trying it;
Shadowrun, Fifth Edition, page 456
The attributes of partial limbs (including cyberhands and –feet) may be enhanced, but their attributes only apply for tests directly involving those limbs (such as a Strength Test when gripping something with an enhanced cyberhand).
The rules regarding location armor do exist; the snake-proof socks from Run & Gun are the primary example off the top of my head. It's a completely by-the-book reason why it wouldn't be effective. Personally, even if they didn't like it, I wouldn't care. There's enough broken in this game at the moment that needs house-ruling anyways, I'd just throw it on the list.
Oh, sure. I agree it's absolutely stupid that Armor 3 from a Hand applies when a shot hits you in the chest. I'm just saying that it is entirely valid by RAW. Who's to say you don't pull your hand up as a reflexive action when getting shot at? Unless you want to tack on a couple thousand optional rules for what your initiative should reduce by when you try to block a shot coming towards you, which most people actually do when getting shot at. So, it is possible that the Armor 3 would apply. I'm sure every GM has their own version of a houserule for it, but for those who don't, it applies and should be considered about whether it breaks your game or not.
Also, as far as what you can put in a partial limb or appendage, there's a lot of options besides cyberweapons. Run & Gun actually statted out the capacity costs for a lot of gear; 4 capacity could hold scanners, lock-picking gear (go-go gadget lockpicks!), a mini-welder, commlink, wire clippers, tag eraser, a camera or directional microphone and a lot of other things.
Scanners require Electronic Warfare to operate. Very few people take that. Lock-picking gear can be useful I suppose, but you can fit maybe one item into your cyberhand. That's really useful when you should be carrying the things around in the first place if you are planning on using them. Mini-welders, well, I don't think I've ever seen a normal one, so I don't think an implanted one is at the top of people's lists. A commlink is implanted in the hand if it takes up capacity, right? That means that you can't remove it easily. You don't want that with Burner links, and an expensive one is most likely going to be implanted in your head, due to the fact that if someone cuts off your head, you aren't around to worry. If someone cuts off your hand, there goes your commlink.
As for the rest of the stuff you listed, sure it's useful. Out of all of those things, there's not a single one I wouldn't take in a different piece of 'ware or just carry. That's probably my personal preference, but I can't say I've seen anyone take a single one of those items as an implant into their cyberlimb.
I wasn't commenting on the relative frequency of cyberlimbs. I do agree with you on that, however. I was thinking of making a Rigger/Sam based off of Dragonlance, of all things, a dwarf with a massive obvious silver cyberarm, and I saw how it'd cost 40k just to get an arm at 8 Strength, pre-enhancement (on top of the attribute points needed so I can put the arm at that level). I lost interest after that. As I mentioned, I was contesting 8-bit's point on the matter, since it appeared to me to be almost entirely meta-driven, and flawed given the utilitarian possibilities.
Well, I'll just say that a Dwarf can get an 8 Strength Arm with Enhanced Strength 3 even if their base is only 3. You can customize to your natural maximum, so you don't even need your strength to actually be that high.
I don't mean any hostility here, but you did not really answer my argument. You pointed out that cyberhands can have other things inside of them rather than just armor; that's true. That was not, in really any way, the point of the argument or post. The point of it was asking whether you think this ridiculous armor stacking is fine for a player to do, or whether it requires the GM ban hammer at your table. The whole "it's meta-driven" seems to imply to me that you are referencing that I forgot you could install other things in cyberhands. My actual argument can never be utilitarian based, because the entire point of it was that someone can put all their money into armor and basically become a nigh unkillable tank. My post was illustrating the fact that
if someone makes this decision to do armor stacking, the numbers can inflate pretty quickly, which is entirely "meta-driven" and in no way invalidates the point of the post. Not to take the specific details and nuances of creating the perfect cyberhand.
So, my hopefully more clear question is, do you allow a person to take 4 Partial Limbs (we'll go with that since it's slightly more reasonable than hands or feet), Titanium Bone Lacing, and a Cyberskull, which can add to a total bonus of +3 Body for Soak Rolls and 18 extra Armor? Combined with a Body 11 Troll and Ares Big Game Hunter Armor, you are looking at 48 dice for Soak Tests. Do you consider this munchkining and disallow it, or do you let a person build a character like this? It is perfectly legal to build this way. Just assume all the cyberlimbs have Armor 3; the rest of the capacity doesn't matter.
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Personally, I've made the rule that you don't get your armor bonus from limbs unless that limb is being targeted, partial or not.
In combat, I always assume you are aiming center mass unless otherwise specifically stated.
How does that factor with holding a gun or sword? You usually hold them with your arms in front of you, covering a decent portion of center mass. Would you roll percentile dice to determine if the limbs factor in? I agree with the center mass part, but there can be a lot of things that are in that, for lack of a better term, "center mass area", including limbs.