I didn't register until a week ago, so I suppose I can give you a first-person answer.

As some of the others have suggested, I didn't make a conscious decision to avoid registering; I just didn't see much point. I'm pretty comfortable with perl and can research my problems, so I haven't had to ask any questions. I'm not drawn to a lot of the community activities (golf, poetry, etc.) & haven't met any of the perl movers & shakers. I don't muck with perl's internals & haven't contributed anything to CPAN. This cuts me out of most of the activity on this site.

So I would typically come to the site to see what's on the front page, same as slashdot, techdirt, and a bunch of other sites I frequent. But I didn't see any point in registering; it'd just be another password to remember for no benefit (I've never registered for slashdot either, though I've probably been checking it a few times a day for the past several years).

I do feel a certain calling to mentor other people. I read usenet a couple of days a week, so virtually all of the good questions are answered by the time I see them.

I couldn't tell you exactly what made me register. A vague sense that I should make more of an effort to get involved, probably combined with a specific question that I saw and wanted to answer. Oh, and I didn't like the default white background, and registering was the only way to change it (seriously).