I believe as a registered user, you have added benefit that the system keeps track of what you've seen and what you haven't seen, that alone is worth registering, even if you only view, never post...
Not sure if it's true on this site or not, but on many similar forums, you can't do searches as well as a guest, as you can as a registered user.
There may very well be other little things that being a registered user gets you... and I think some people are just shy or reluctant to post, for whatever reasons... nothing necessaily bad.