First, some facts about the T-rex.
While the arms were pretty much useless to the beast, they weren't as weak as you would think. They each could lift about 400lbs. Nothing compared to what it's legs could probably do, but that's not bad.
As for what they were used for? Well, so far scientists think if they were used for anything, it was sex.
Basically, they think the T-rex would use it's hands to "tickle" it's mate by gently scraping their scales.
Also, one other little know fact about the T-rex is that they were all killed by the Kaiju. ;D
My Jaeger's name is "Bad Rapids". Super weapon; "Slam Skin".
I don't get hurt much. Almost never. The worst I get normally are cuts on my arms, legs, and pretty much anywhere else that a sharp thing can get to and cut me.
The worst injuries I've gotten are where when I fell and got lots of my skin scraped off, but it all healed and nothing else got hurt. So for injured body part, I'll be going with skin. As for what the weapon is. Well, I guess since it's my skin, it will have to be more of a mode.
The jaeger's "skin" has spikes come out of it, and they set up an electric field. It's called Slam Skin because as jaegers, that's what we do. Hit the Kaiju until they're dead. This will be good because if they try to grab Bad Rapids they'll get cut up, and they'll get electrified if they so much as touch it. Also, I would imagine getting hit in the face with a spiked, electric robot fist would hurt.