I really like charging into an enemy and then shooting them/another in the face with my shotgun. Holstering would really cramp my style... and the fun.
Would be funny if they had a casting system like DAO where some of the spells required you to sheath weapons, and some did not.
Holster weapon button could make a comeback though. That is if combat and non-combat stuff takes place in the same areas.
It would be worse if we had to stand completly still to even use our biotics like with magic. What would even be the point then?

Don't mind being able to holster when not in combat, though.