Also note that with something like AP, we're talking about more than one column on last page's chart. So while the diminishing returns for a single hit is pretty apparent (2->3 is +14.81; 3->4 is +9.88), as long as the total armor is equal to or greater than the AP value, your variance/standard deviation is going to increase with each additional point of AP. -1 AP has a 33.33% chance of taking one hit off the table. -2 AP has a 44.4% chance (not 66.66%) of taking exactly one hit off the table, but it also has an 11.11% chance of taking two hits away. So you have a 55.56% chance of doing something, but your ceiling is higher. Looking strictly at game mechanics, I think there's an argument to made for increasing your variance since damage has certain meaningful thresholds (wound penalties, death, etc.), so not every box in your condition monitor has an equal value. Now it's probably worth your time to cost the different forms of advancement in the game to maximize your ROI.
More specific example:
You want to be able to reliably drop an average opponent with your Predator with two shots, so each shot needs to do 5 boxes of damage. Finger waving the attack and defense rolls, we'll say you can reliably get 2 net hits, bringing your DV to 10P/-1.
Vs. a Body 3, Armor Jacket opponent, this will get you 5+ boxes of damage 47.55% of the time. Not as reliable as you'd like.
Now, let's imagine APDS is available in different grades.
Grade 1 (10P/-2) gets you to 55.2% (+7.65).
Grade 2 (10P/-3) gets you to 63.15% (+7.95)
Grade 3 (10P/-4) gets you to 71.1% (+7.95)
Grade 4--actual APDS--gets to 78.69% (+7.59)
Grade 5 +5.69
Grade 6 +4.26
Grade 7 +2.75
The overall process does involve diminishing returns--Grade 7 APDS turns out to be a bad investment--and you can get a mental picture of the curve based on these values. However, adding to the armor value--let's give him a helmet and forearm guards and some PPP kits--moves these numbers as well, essentially shifting the drop off point to the right. Not surprisingly, higher levels of AP are more valuable against more heavily-armored opposition.