Validating E-Mail addresses prevents most users from making multiple accounts, and inflating your statistics. If a user sees that he or she has registered an account with their E-Mail address, then they'll request the user/password via a lost information form.
If you do not validate E-Mail addresses, there's no logical way for a user to recover their login information.
It does not stop bots. Bots can be intelligent enough to retrieve E-Mails from SMTP servers, and access the validation links. E-Mails can be predictable due to the minimal variety of forum software forum boards use. Captcha is the way to go there.