Food for thought: 95% of the clients I build sites for **demand** a CMS.
Of those, 95% of them email me to update their site. They never really use it.
If you have multiple users that have no technical experience, it's probably a good idea. If it's just you and you have the skills to maintain the site, don't bother.
- CMS's have vulnerabilities and are often hacked. This is really the #1 reason to avoid a CMS unless you need to use it, another potential hole in security. Static files can't be hacked (if your FTP and server are reasonably nailed tight.)
- Many times you will spend much more time figuring out how to get chunks into a CMS in some useful way that can take hours to figure out, but you could have it coded static in five minutes.
- CMS's are all about templating and automation, which often gets in the way of unique instances. Just the other day we had to revise an entire method because a single instance of a templated set of pages required one unique instance. I mean literally, one subheading had to change - this requirement was not in the original setup of the CMS. It took three hours to make this happen (to get it "editable" in the CMS.) For one word.
- SEO-happy - usually you can tweak a CMS to be SEO compatible, but often there are compromises. I haven't encountered any I couldn't solve, but can see how many wouldn't be able to. It's often a struggle.
- Bloat - there's no way any CMS driven site can load faster than a static page. I'm just not convinced.
- Long term costs vs. short term costs - Weigh in the frequency with which the site is updated and that cost over time versus the cost of implementing a CMS, making it work as well as the static pages, and whether that investment will balance out over time against maintenance costs. If you actually put this on record, many times it's cheaper for the client (or you as developer) to avoid the CMS.
Not po-ing CMS's . . . these are just some factors to consider when making the call. I use them every day, of many different brands and flavors, just most of the sites I use them on don't need it.