It's kooky. You average all limbs involved in a test. The example doesn't work if you treat Cyberskulls as a full limb (but makes it unquestionably clear that cybertorsos are full limbs). The other way to read the text is that you average all your body scores, ie, each cyberlimb once and your own score once, regardless of how many limbs you have, but that is considerably less popular of a reading.
At the end of the day, the Platonic Form of the cyberlimb rules doesn't matter, just what your GM rules they work like.
However, "all limbs involved in a test" does lead to stuff where your test doesn't involve your ware at all. For example, toxins - there's no interaction with any of your cyberware, so if you are exposed to, say, poison gas, the body test should only use your meat body score.
My stance is that General Grievous is a perfectly valid way to roll. Yes, you DO have some weaknesses (like toxins); if you're smart, you'll prepare for them. You also pay a lot of nuyen and essence, and you have the disadvantage of looking very unmistakeably like a killer cyborg. In return you are in fact a killer cyborg. Generally, characters like this wind up trading offense for defense; you aren't going to be able to afford quite as much IP boosting or whatnot as a differently focused street samurai because of all the essence used on the ware, but you will also be a lot tougher.
