You might think that everyone would choose a site with a CMS so they can make their own changes whenever they want. Well, some clients do. Some people enjoy maintaining their own site, and there's no arguing it will save you a little money.
But there is something very important to take into consideration — time. Specifically, your time. Even a CMS designed to make keeping your website up-to-date takes quite a lot of time ... time you would probably rather spend running your business or organization. Add in the learning curve getting up to speed on the CMS, itself, and suddenly you have another part-time job.