There are legitimate reasons for introducing hurdles like requiring registration, requiring activation e-mails, etc. Ask yourself whether the benefits of activating outweigh the potential downsides. If you're requiring activation e-mails to prevent bots from signing up, how significant of an impact will those bots have on other users? On a forum it may be pretty significant, as the site can be flooded with unwanted posts. There are other cases where a bot signing up would really have no substantive impact on a user, because they wouldn't be inconvenienced by the bot.
That's a rather roundabout answer, but it effectively boils down to this: you should remove every unnecessary barrier between your user and your site. I consider an unnecessary barrier something that doesn't improve the quality of the site and/or the user experience.