It's down to security issues but to be honest, if a site requires you to activate the email at least they are trying to prevent spam on their end making the forum experience better for you when you don't see as many spammers. Agreed that on small sites it might not change things but it's a precaution because you can't tell if something like spam and what not will ever happen or when.
I've never used email verification on any of my forums. I did have a huge problem with spam accounts and I installed re-captcha and I haven't since (touch wood) had any problems with spam.
In my opinion a small board would have trouble gaining members because lets be honest, some of us yawn at the fact that we have to activate our account through e-mail. It sure is lazy and its an extra thing to do. People can't be bothered and are to inpatient.
I think as your user base grows and you start attracting traffic elsewhere, spam will start to pick up and the e-mail verification should be pursued. By this point, content should be of high quality and people should want to sign up to your boards regardless of a e-mail link.
I just haven't reached this stage with my community yet.
