One the common concerns in using open source software at some companies is having formal maintenance and support contracts. The crazy thing is that most enterprise software products come with really bad maintenance and support at high prices.
Take for example Premier support offered by most enterprise software vendors that include claims of 4 hour response times and the like. If your goal is having someone on the phone, then sure this works ok. However if you actually want your problem solved, then get ready for a surprise. Most of the time you are going to have to pull some pretty intense efforts to prove the issue is the vendors problem. In most of these situations you end up doing all of the leg work which often includes writing the fix or workaround. In many cases, the vendor boils down to becoming a place for you to file bugs along with their solutions so that you dont have to deal with in the next release.
Upgrades. Oh don't get me started. Enterprise software vendors never sell licenses on how easy it is to install or upgrade their software. In fact, sometimes I wonder if they don't intentionally make this process difficult in order to require customers to buy more hours from consulting services.
Lastly, the enterprise product knowledge bases often require you to use the vendors website using customer logins. This limits who has access and makes searching and contributing to this content difficult if not impossible.
Open Source on the other hand has to have great maintenance and support processes in order to gain any traction and acquire a user base. Wordpress is a great example. The installation and upgrade process is as simple as it gets. Searching for any problems or issues is as simple as searching the web.
Like many things in the world, the logical choice is not the first choice due to tradition and fear. Just keep in mind times are changing, walls are falling, and open source will set you free.