Its only really useful if you want to storm an enemy base but leave the buildings standing (so you can complete research or accululate power before the timer runs out).

The only reason I put this as a bug is because if the take a group of units, set them to return fire, then send them at the enemy, then assuming they survived, some will still set to "Return fire" and some will be set to "Fire-at-Will". While this may or may not be the intended behavior, it is not , in my opinion the correct or usual behavior of a "Return Fire" setting in rts games. I think they should keep their "return fire" setting even after they get shot at. (And still fire back of course.)

This is something I've noticed while beta testing the campaign, If you set units to the 'Return Fire' setting they only hold fire until they are attacked, then set themselves to 'Fire-at-Will'.

Giving an attack command does not cause them to switch to 'Fire-at-Will' nor does firing their weapon, however getting attacked causes the units to forget their 'Return Fire' setting completely and instead use 'Fire-at-Will'

I think this may have been around for quite a while and just gone unnoticed as there is not many situations you would need, or want to use 'Return Fire' outside of the campaign.