Agreed. There are a number of cases where a valid, authenticated email is a requirement to use the system (for instance, when other users will be using that email to find and link to you). In those cases, an activation email is unavoidable.
Daniel
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.