I do think that sometimes people add sitemaps (xml) to solve problems they haven't got, just because they've heard someone mention on a website that adding a sitemap will improve ranking. If your site is already search engine friendly and being indexed by google then you don't really need a sitemap. Some will say that sitemaps can help since they can tell the search engines what pages have changed or have been added, but surely shouldn't you be telling your real users about this too (e.g. having a latest news links from your home page, which will get picked up quickly by search engines too), without this you are merely pandering for the search engines not for your users - something that google tells you not to do.
Another issue with sitemaps is that some people wrongly think that it's a one off setup job, get some tool to generate a sitemap file, add the xml file to your site and that's it. This merely gives a snapshot of your site at a given time, and your sitemap will become outdated as you add to your site, thus giving the search engines conflicting views of what your site is all about. So if you do generate and add sitemap files, do it on a regular basis as you update your site.
The quality of some sitemap generation tools can be questionable too. Give a sitemap generation tool your web address and get it to generate your sitemap, and you are then relying on the sitemap tool being able to find your pages and hoping that your site is search engine friendly. A situation where a sitemap is a 'must have' is when your site isn't search engine friendly, but if this is the case, then an automatic sitemap generation tool wont be able to find your pages either. With a partially search engine friendly site, there's more chance that google will find your pages via its own bot indexing than some sitemap generation tool, since google's indexing algorithm will be more advanced.
So a sitemap that is wrong, for any reason, will be detrimental to a site, and this issue has
cropped up before on this forum.
Finally one of my clients was approached by a 'SEO consultancy' peddling sitemap snakeoil, and I was asked to add a sitemap file the SEO consultant had created for the site. My client soon ditched the SEO consultant when I pointed out that the sitemap file they supplied was for a completely different company's website!