This isn't an uncommon question that we run up against in most of our prospective client meetings. It is actually one of my favorite questions to answer, not only because it means the client is doing their research into the content management industry (or having another vendor present a different solution), but it is relatively easy to make a strong case for the Software-as-a-Service model.
First, let me just say that the majority of the time that Open Source is brought up, the client is referring to one of three CMSes: Wordpress, Joomla, or Drupal. Each one of these systems has its strengths and weaknesses (I promise they all have downsides, no matter what that programmer tells you). These systems are incredibly flexible and can seem to have an endless amount of plug-ins that can be utilized for increased functionality. They are typically free from any sort of subscription fee, which is also very enticing.
Software-as-a-Service Content Management Systems, on the other hand, are often times proprietary systems that rely on a single company's efforts to expand on the product functionality, which can be a bit limiting at times. These systems, many of which have been in development for years, can also be incredibly powerful and flexible though. The main difference, in the eyes of the customer, is that "pesky" subscription fee.
If you're a small business owner, marketing director for a mid-size company, or the Chief Marketing Officer for a Fortune 500 company, you just want your CMS to work when you need to update your website. You probably don't care about the technical specs, or how many developers contribute to the vast database of plug-ins, you just require simplicity and reliability. My question to you, then, is "How important is support?"
Support is where Software-as-a-Service differentiates itself from Open Source Systems. If your Open Source CMS system breaks, who can you call? You can't call the developer of the plug-in that is broken. You can't call Wordpress or Joomla. You have to call whoever built your website in this system, and I'm willing to bet, the time required to fix your issue isn't free.
A few SaaS CMS platforms come with unlimited support for the system. If you have a problem, you can call the programmers who built the system and get it figured out. There is no searching the web for help, scouring documentation that, depending on your technical skills, might as well be in a foreign language, or digging into your bank account to pay unexpected support costs. Think of SaaS as a true website partnership with a CMS company.
With all of that being said, no CMS is the right CMS for every project. Since this post was primarily about supporting a CMS, I didn't get into detail about specific pros and cons of each type of system.