How much content do you have for the site? How much stuff do you have to put on your web site? Check your brochures, info sheets, catalogs, and sales write ups.
How many pages of data do you have? If you have less than about ten pages of content (such as home, about, services, and contact) then you probably don't need to use a CMS.
How frequently will your content, or data, change? It's much easier to modify and change a site when the site is done in a content management system.
However, there's a greater initial time, expense, and effort cost to setting up the CMS than there is with a simple site.
If your prices change daily or your products change weekly or you highlight new people regularly, consider using a system.
If you only plan on touching up data once or twice a year, a straight HTML site is probably fine.
Will you sell stuff online? The popular content management systems have extensions, or widgets, available that create eCommerce systems on your site.
These extra programs are relatively cheap compared to the cost of writing a new system from scratch.
Do you need your customers to have accounts with unique logins? Do your customers need to be able to log in to your system to access special data? Do you need a membership site? Similar to selling stuff online, there are extensions and widgets available for the different management systems, so you can pay a relatively small amount of money to set this up, or you can re-invent the wheel and pay a lot of money and time to create a new membership arena.
Does your site need a forum or social networking? Do you need a forum, like for tech support or customer service? Do your customers need to be able to "friend" each other? Does your online presence need to be integrated with other social networking sites like Facebook and Twitter? Again, this is easier done when you use pre-packaged software, rather than writing stuff from scratch.
Do you want to change the look and feel of your site quickly or frequently? Separating the data from the formatting is one of the main goals of a typical Content Management System, so if you want to change the colors, fonts, and graphics on your site quickly, regularly, or frequently, it's a good idea to consider these well established tools.
Do you need to log information regarding who makes what changes to your site? WordPress, Joomla, and the like have administrator accounts with various levels of permissions.
This not only allows you to limit the power some people have over the site, it also allows you to track who has done what.