A sitemap is a good way to tell the SE's about your pages, but they will get indexed faster if you link to the pages from other pages that are already indexed.
Just think about it this way... the SE's crawl links to find pages. If you have a page that has no incoming links then how would the SE's find them? And are they of any value if there are no links to them?
By the way, I have an entire section of one of my sites that is for my clients only so I don't link to any of it publicly but the pages were in my sitemap (by accident) and they were not only picked up but did well in the SERPs. So I had to fix the sitemap and block the SE's from those pages to keep them hidden. ... just fyi