Using the rel attribute is kinda a win lose situation. You can use it for stuff like the “lightbox”, but it won’t validate. Not validating isn’t the end of the world though.
WordPress has a rel attribute hard coded in it that doesn’t validate. Does that mean WordPress is wrong? No, it just means it’s not going to have perfect validation.
As long as you don’t care about having your site validate perfectly go ahead and use the rel attribute as you see fit. Just don’t go overboard. Only use it if you need to. Most of the time you can get away with classes and id’s rather than rel.