A necessary add-on if you are using Amazon SES. Plus it allows your moderators to help with some of those mundane emails you probably receive on a daily basis. Especially useful with Yorick's code changes within the discussion thread to utilize multiple thread pre-fixes to categorize the contact requests.
So very useful. Allows your staff to address issues, rather depending on your checking e-mail. Also good to prevent spam from reaching your e-mail, for those who would abuse the contact us feature. And finally, limits out going e-mails. Should be standard in XenForo.