well you'd hear the crit sound and someone's death screams, the killfeed goes off and the casters lose it. it's even more obvious if it's the only kill going on in a stalemate

on the other hand in a team fight with frags going down all around a bunch of killsounds is definitely not going to help anyone.

I think the game only lets you specify one killsound so hearing that same sound every time any player died might be somewhat confusing and could cause some weird anticlimactic moments where somebody dies on flank and you think that it was a sniper pick for a split second. Maybe if there was a way to specify a killsound for each class or even for each class dying to each other class it would work but that seems like way too much work not only from Valve but from a producer/cameraman just to get the sounds working and then enable/disable the killsounds on the fly in appropriate situations.