My feelings on this is that getting assist kills in first person shooters today isn't rewarding enough. I've played games before where I've gone like 2 kills 8 assists and 1 death. It's almost like getting an assist kill is bad because you don't get any reward for it and most of the time you've exposed your position to the enemy.
Perhaps even have separate different support bonus for assist and tag kills, maybe like support bonuses that help your team. But when you get the full kill streak you get a nice assault bonus that adds to your kills.