Yeah, I know, they really do. I was meandering about in Yelp and clicked on find friends. Yelp wants full access to my Gmail account, with which they would do ... something.
It's not clear if they would tell me which of my contacts have yelp accounts, or send them e-mail, or what. They are clear that they do not store my Gmail password, but say nothing else about what they do. And since many online providers have been penetrated, and since many online providers mislead users about how they respect their users' privacy (see Google's recent subversion of Safari security settings for a recent example, or Facebook's historic lack of regard for their users), it's foolish to give one service complete access to another service.
So with my Google password comes access to Gmail, Blogger, Picasa, Docs, etc. Nope, not giving that out to another service, and especially not one that doesn't explain it's intended use for said password.