The problem is that hitting those arms is the same as hitting a power armored person, not as easy as you think, even wasting your time in the end, because its also as thick armored as the whole body. Especially with the fact that the big arms is always get in the way (look at the Mk12 Gecko, you can't even see those small arms on regular basis). Also knowing that TAGs is always on the move makes hitting such small targets an even harder task, even more so if you're constantly under fire. You can use the missiles, but you'll end up destroying the entire TAG anyway. So those exposed small arms are pretty much a less problematic concern.
