My question would be "why make a sitemap" -- but I've said for close to a decade that if you 'need' a sitemap.xml there's something wrong with the site it's being applied to; since the links IN the site should provide all the 'structure' you need to be crawled... if it isn't, well...
There's a lot of 'extra metadata' garbage like sitemaps that serve no legitimate purpose other that to sweep developer ineptitude under the rug, and seems to exist out of ignorance on the part of who came up with them on how things should work. See Microformats, the new allegedly (and decidedly not) semantic and uselessly redundant HTML 5 tags, Ruby roles, pointless title attributes identical to the text inside the tags they're applied to, etc, etc, etc...
Really if you 'need' a sitemap, you've done something wrong.