If you mean logging in via clicking a link in an email then it is too much of a security risk. Think of all the places that URL can end up in: browser history,

If you mean logging in via clicking a link in an email then it is too much of a security risk. Think of all the places that URL can end up in: browser history, bookmarks, etc. It works for password reset because the window of opportunity is small (just a few minutes) and it is a once-only operation (visiting the URL again should not reset your password again).