If I may express an opinion (which may have been said already but..... 13 pages =|). =)
To be honest, I cannot see how such hard and fast rules about what to kick or what to not kick can be made at all. Every class has its moments and weaknesses. Some are more effective against certain enemies than others, but so long as the user of the class is skilled enough, there's no reason why the use of such a class is invalid or kick-worthy. For example, a good human vanguard can do wonders when they charge around the field, setting off biotics (if available) and reviving their teammates. Sure, there may be times where they get taken down, but that's why the vanguard is always called high-risk-high-reward. I cannot think of a class that is completely useless enough to warrant it being kicked from a game. Perhaps that class isn't as effective, but that can be made up by the user's skill level.
You may say then, well, why take that risk? Why risk having a bad player on our team with a less effective class? To which I would answer: I don't feel I have the right to judge someone before I've actually seen them play. More generally, I don't feel I have the right to deny someone else to have fun with whatever class they choose, just because I may lose the game. So long as we have fun, it's all good for me.
The other point is this: if we agree it is player skill level that matters, then how can we determine this level? To which I would say: you cannot, at least not until you get into the game and see them play. The N7 rating doesn't tell you anything other than the fact that the player has played and promoted his characters many times. They may be good through experience and sheer exposure, but they could also have gotten to N7 2000 playing exclusively on bronze and not understand gold. It doesn't tell you whether or not that player is a good vanguard, or a fantastic engineer, or that he's just good at the game. Furthermore, a lot of players do not promote at all. I've racked up something like 40-50 hours on ME3 multiplayer and I'm still only N7 170-ish, because I've only promoted twice. Do I deserve to be kicked because I'm sub-N7200? Using the N7 rating as the criteria by which a person is judged useless enough to be kicked is just largely pointless.
What about the character's level? Should a lvl 1 vanguard with a predator be banned? Here you may be able to set some lines as to when you really shouldn't be in gold, and when you should. Having said that, there's a video of a couple of Russian Krogans on Lvl1 finishing gold. I'd hold them as the exception, but it still goes to show that a clear line cannot always be drawn.
Bottom line is, you cannot always find such hard and fast definitions as to what makes a player or character so bad that he or she needs to be removed from the game before it even starts. I don't really have an objection to kicking a player after a game, if they have been atrociously bad. But before the game has even started, before you can validly judge the player's skills and effectiveness, surely it's unfair to the other player to kick them from the game?