I think reload cancelling would be considered an exploit because you are altering how the reloading mechanism is inteded to work. The developers certainly didn't intend to fire the guns in the middle of the animation. Just think about it....the cartridge case is ejected when you pull back the bolt handler. If at that point the round appears in the barrel but at the same time the animation is not finished yet (e.g. the bolt handler hasn't been pushed back in place") the gun wouldn't be able to fire because the next round wasn't put in place properly. It's unrealistic. (I know, it existed in myriad other games before but in most games you have to actually move for this to work. Here you just press a power off-cooldown, you don't have to leave cover.)
I think if ME3 multiplayer was pvp this shouldn't be allowed to exist. As it stands, it increases your team's damage, so it's only a minor annoyance.
Problems, I think, only arise, when people take extreme advantage (abuse) reload cancelling. For example: human soldier with maxed Adrenaline Rush and Claymore shotgun. Fire the Claymore, then pop Adrenaline Rush to instantly replace the round, THEN use reload cancelling. (3 shots fired off in a very quick succession from a shotgun that is not meant for this.) I've seen a few people do this, not many, but I consider it somewhat an unfair advantage to their DPS. The Claymore was designed with a slow reload animation for a reason. (Same goes for the Widow and whatnot.)
Of course experienced players who use reload cancelling frequently usually have far better weapons than the new players who haven't heard about this feature. The massive difference in damage would come from the fact that one person has far better weapons than the other and not from the fact that he is reload cancelling. (some people said otherwise)
It would be good if they fixed this but I'm not concerned either way. I'd like to see however the invulnerable cloaked Geth Hunters fixed. We had to leave two gold matches today in rounds 7 and 9 because for some reason two Geth Hunters became.....invulnerable.