The answer, as with many things, is it depends. Pre-built CMS tools and custom software each offer a lot, but also have their drawbacks. It's a matter of weighing the business needs vs. what you are getting.
Pre-built CMSs (especially those that have been around a while), are usually pretty easy to use. They allow your clients to update their own content, taking maintenance of that off of a developer's plate, allowing them to work on other things to add value to the business. CMSs usually have a lot of online support, both for users and developers, meaning most problems can be easily solved. CMSs usually offer a lot of features that will cover most of what a small business needs. They can also be set up quickly, allowing a company to get something out there quickly (this can be crucial if time to market is important). They are also typically pretty cheap (compared to custom software).
On the flip side, CMSs will have a bit of a learning curve, both for the users and developers. There is also a chance that it might not have a feature you need. It may exist out there as a module or it may not, requiring someone (read: a developer) to make it. That could be easy or it could be very difficult. If a CMS doesn't meet enough of the business's requirements, it could easily be more cumbersome than building custom software.
The nice thing about custom software is that you can get everything you want and nothing you don't need. Things can come out exactly as you want, with processes and features tailored to a business's specific needs.
But all that custom fit niceness comes at a price. It's slower to get up and running. There isn't any online support for users. It hasn't been used and tested by many, many people (meaning there are probably bugs). It also ties up one or more developers' time and salary to maintain the application. This can make custom software expensive.
Whether to go the pre-built or custom route is a business decision. If the pre-built CMS does a better job of filling the business need, use it. If the custom built application makes the most business sense, go that route.