Yes a bad team will screw up a bad idea, but if it was as simple as just backing good people then VCs would just give a bunch of cash to a good team and tell them to go out and find something. YC tried this and it was a failure.
What I took from it: if a team can't come up with an idea themselves and rally round it, there's something wrong with the team.
It needn't be a very good idea. e.g. the reddit guys were excited about a delivery service idea (pet food? muffins? IDR). pg said no, and suggested the reddit/HN idea. But note that they had come up with an idea and rallied around it on their own. (Who knows, perhaps they would have made it work anyway?)
Maybe it's a touch of mad passion/enthusiasm/optimism - not entirely rational - that is a key ingredient of an entrepreneurial team. Indicated by inventing and rallying round "some idea".