Lol @ MyBB 1.6.5 hidden "CAPTCHA"

[ZiNgA BuRgA]

I've had plenty of (legit) registrations in the past who never post.  TBH, I'm not too sure why a lot of people do it - maybe it's for the ability to change settings or remember read threads, or maybe they intended to post, but changed their mind later on.

You can check if they've filled in their signature or website - if so, a good possibility of a spammer.