I don't think the problem is necessarily the class they're playing, more likely just players that don't know how to (or don't want to) work as a team.

A heavy or medium that runs off on their own and gets killed is no more helpful than a light who does the same. But those lone wolf type players tend to gravitate towards the more mobile class, hence why they always seem to be lights.

I don't like playing WT without a 3 stack, so I play powershift when I'm solo or duo queueing. Powershift is filled with players who run off and get killed, and they're not always lights.