Yes you can but a normal XML sitemap is not intended for users and not everyone is a technical expert. If you want to, use a XSL stylesheet on your XML sitemap so that it is readable to users. Search on the net for css for XML and I'm sure you will find resources on it.
no it is not at all useful. your visitors can't understand it. it is better to place the URL of xml sitemap in your robots file.
Yes,Its very Useful for your site need rank on any search engines.Google search engine crawl your sites sitemap pages which is site.xml file. and yahoo crawl your sites sitemap pages its name is urllist.txt and for MSN is sitemap.html. this all are in backhand for your site.its understand only technical person.