That's OK if you trust the site as much as you trust your email provider. There's still a mild risk that they'll store data that they obtained when they logged

That's OK if you trust the site as much as you trust your email provider. There's still a mild risk that they'll store data that they obtained when they logged in to screen-scrape for contacts and that data will leak. It's probably worse in gmail and other services that show potentially confidential snippets of emails as soon as you log in.