It really does depend on what your site is going to be used for. If you are going to be doing regular updates, then it is probably a good idea to have a CMS system. If you find yourself spending a lot of time updating your static web pages, or are finding it increasingly difficult to manage, it is probably a good idea to update to a CMS. The question these days should be why shouldn't I use a CMS system? Even for 2-5 page sites, managing the content of the pages using a Content Management System makes life a lot easier.
