It is an obvious need.

A good machingunner should fire 99% of the time (just a number) while resting on bipod.

Also when deffending fortified positions everybody rests their weapons while firing most of the time.

I guess they are trying to make it perfect (static point of rest, animations, AI implementation and so on) and because of that they hadn't released it yet. What I spect is that if they don't have the time to do it the realistic way they will release it as just a button that reduces your recoil while static over a point of support.