After several years of developing websites and working with clients I have concluded that at some point in time the client will out grow the site. So, instead of having to convert a basic HTML/CSS site to Joomla why not just do it once and save a lot of time and effort later.
The biggest request I receive is to be able to update the site when needed. Many times it's just a price change, swap out an image, etc. so building a site running on a content management system such a Joomla solves their requests much more efficiently. It also provides the framework to add a forum, social feeds, photogallery, or slideshow much more quickly.
Another advantage of using a CMS is the ability to create a special feature such as the one on the French-Canadian Genealogical Society of Connecticut's website. Which is used to add Surnames to a database that the members are researching and the ability to find other members researching the same family as themselves.
Those are some of the reasons for my decision if you have any comments I would be happy to hear from you.