This question comes up from time to time, and the answers are usually fifty fifty as to don't do it, it's wrong, and sure, anything to save a buck.

If you buy a ticket there's really no issue, as you are a convention attendee even if you don't step foot in the convention.

I wouldn't do it if I was not registered for the convention, you may get caught if they cross reference, and you may keep somebody who should be going to the convention from staying there.