I wont lie, I've been in a comp game that someone else has picked what I would like to play and I'm 'forced' into a support or tank role. I don't mind this if the team is then working, but it's not enjoyable when people that have taken that character can't fulfil their role.
Example, based on what I said above, I was playing yesterday and both DPS went very quickly. I chose Lucia, a support healer.
The enemy had a very good Phara, which shouldn't be a problem, we had a Soldier, Phara's counter. Unfortunately, no matter how much we asked him to focus on Phara, he barely ever even shot at her, kept saying 'I can't see Phara?'... you couldn't really miss her, flying around over our heads.
I probably should have asked him to simply switch out, but in the end our other DPS switched to McCree and started doing Soldiers job for him. We lost the game though. 20 minutes of annoyance really, but you can't just leave as it's a Comp match.
If we'd been a bit more forceful, asked him to switch and he wouldn't, I probably would report him.
If you play Quick Play, pick whatever you want , unless someone already has them. People don't really moan, they might say now and then 'we need a healer' but that's it. Do as you wish.