If the game was close you'd play for the team either way because then there's a chance you'll win and gain points, rather than lose points.
Would you? If it's close and you want to min/max, then you'd rather make those last few kills steals, just in case?
I mean, there's some threshold at which you start playing in a degenerate way. Maybe it's a 10 point lead, maybe it's 5, and it's going to be different depending on the type of player you are. Some players lose hope a lot sooner than others. Do you want those players on your team losing hope at a 5 point deficit and switching to kill steal instead of assist mode?
Do you want this whole new selfish metagame worming its way into team slayer?
I'm not saying which way is right, I'm just pointing out what would happen, and what designers would ask themselves.
Oh, and thanks for having me