I want to do this in order to make it easy to update the site using plain XML files (instead of using a full-fledged & expensive CMS system).
Er, what? There are many free and open-source CMSs.
What spoulson said above about templating is the point. Providing output pages from input XML files is fine. But that doesn't get you a CMS. What about C.R.U.D.? Search? Page listings?
If your site is so small that all the relevant updating can be done by you hand-editing some XML files, then why not, go for it. But if it's that simple you might as well just edit HTML by hand.
