If a site has taken great care to ensure that all pages of the site can be found either from the home page, or from a subcategory page linked to from the front page, would adding and maintaining a sitemap be a waste of time ? My understanding of sitemaps is that they're designed to help search engines find pages that aren't otherwise linked to by other pages on the site, and have no benefit beyond that.
From an SEO standpoint (of which I'm not an expert by any means), it couldn't hurt, I suppose. Plus it's great for those users who just can't seem to figure out a simple website
But in this case "couldn't hurt", actually could hurt if there's no additional benefit. It would mean I'm doing work for no reason. I'm not so sure about a sitemap.xml file being of any use to confused visitors, I would hazzard a guess that they'd be even more confused by the XML format of the sitemap.