Well I've only played on a serious team once, the rest are with friends and there is no drive, it's just to have fun.
On the one serious team that so many refer try to bash I played on .. I'll give a storyboard to it :P
A guildie of mine (Maintain) who also had a high rated 5v5 team had just lost their Shaman. They put a post up on various forums looking for a new one and someone noted they were interested but they would not be able to transfer until the following week. After various trash talking and going back and forth I agree to help them out for the week, they only needed 7 games to get their points for the week and explained I would not be staying.
The team rating was 2300, and we were matched up against another top 5 team that was also rated 2300.
We ended up 3 wins and 4 losses, the wins were only after I told them to change their strategy as it wasn't working. After the games were done I left the team and ended up playing both teams the following week stomping them properly.
so my only experience playing on a serious team is at 2300 rating and we still pulled almost a 50/50 split against a team that was ranked higher than theirs. As for getting kicked off the team blah blah, everyone knew I wasn't staying - just helping them out. I suppose any way you look at it though they can spin the story, probably shouldn't of helped them in the first place :)