For me the animation makes perfectly sense. Imagine yourself shooting off missile after missile from your underarm. There are a couple of problems with this:
- To have a recoil-less firing you have to let the exhaust out of the back of the firing mechanism, that's straight into your face with your arm straight and aiming. (Animation wise)
- Even if recoil-less its never completely still and your arm will shake somewhat. After a while i would imagine one becoming seriously tired in the arm.
- If not recoil-less the problem will be even worse...
On the other hand having it fire from your back you negate all of the above negative effects. You take the brunt of the force on your strongest part of your body, legs and back.
Fusion missiles and Explosive darts are only fired now and then and are thus manageable to shoot from your underarm.
Good work Bioware, you got it right on the first try!