I cant count how many times I got killed by phantom when playing arty and trying to support my team from range. Or how many times a single sniper, perfectly positioned could stop my team pretty much by himself. They have important roles when used for team.
It is quite simple, any merc can by played for team or solo. It is up to a player to play the merc the way he wants.
I try to play my arty for team, which more than it is healthy, makes me negative in K/D. But I dont care about K/D as long as my team is able to supress enemy and make a move on objective. Could I have a positive K/D ? Sure, but it is more about a team than myself.
Negative K/D might suggest that I am useless for team, but as an arty, I try to keep enemies at bay as long as I can for my team to advance and finish them from close range. That is my way of playing and it shows sometimes, sometimes not.