It’s a good idea but you still have to go to their site, find the login and type it in. I really am fed up with sites that require a registration, I usually just don’t bother. It’s really unlikely they have anything I will want to see THAT much.
Also, any site which you give your email address to can sell it to spammers or spam you themselves. If you ever wondered how you get so much spam when no one has your email address, registering for sites is one of the ways.