Meh, instagib is still very possible without /assist - I PvP in a couple MMOs without an assist function, and trust me a good PvP team will follow the calls and instagib the entire opposing blob in quick succession.
It *will* prevent less experienced/aware PvP teams from executing instakills, but it won't really help against experienced people, *especially* since tab-order is the same across clients anyway. Maybe if tab order were randomized for each person it would be closer to a fix, and require a bit more brainpower.