Why is Free and Open Source Software (FOSS) great? Because it lowers the risk software presents to your business. Here's why:
With FOSS, you have the power of choice, and the freedom of action. Consider a standard scenario for any widely used bit of non-free (call it commercial, although that's not always the case) software. Let's say there's a bug in it that affects your business and that can only be fixed by code changes. In this scenario, you are 100% at the mercy of the supplier of the application. They may consider the bug unimportant, or a low priority compared to other things they want to spend their development time on. Or they may decide it is a bug, and that they want to fix it, but you need to wait for the next scheduled release in 6 months time. It is possible that they might supply you with a quick fix or beta version that fixes the problem, but frankly, that's a rare thing to see, and it's all up to the generosity/flexibility of the supplier.
Compare this to FOSS. Again, let's say there's a bug that affects you, that can only be fixed by code changes. In addition to reporting it to the project maintainers and hoping (roughly analagous to the non-free scenario) you have the option of fixing it yourself or paying someone else to fix it. If the fix is generated by the project maintainers, you then have further choice available. You can wait for the next release, or you can cherry pick that patch from their source code version control repository and apply it to your current version, or run the current snapshot version being developed.
Yes, there are costs and risks to taking these choices, but the important thing is, it's up to you to make that choice. You can weigh up the cost and risk against the benefit to your business. It's up to you to decide, not an employee of another company that will by definition almost never have your interests as their priority.
There are other possible risks as well. Consider a critical piece of software that runs only in the presence of some sort of license server with time-limited licenses (I'm thinking something like AutoCad, although there are plenty of others). What do you do if that company goes out of business? Or, if for some reason, they decide not to sell you licenses anymore, perhaps for legal reasons, or maybe just because they don't like you, or they've changed resellers and the new ones are numpties? If your business is based on this software, you're up the proverbial creek without a paddle. You could find and run a cracked version, but that's illegal and ill-advised. If your business depends on the data locked up in an application specific format that is "proprietary", you're screwed.
Compare this to FOSS: Your license is perpetual and never expires. If you want to run a version that's 10 years old because it works, does everything you need, you're fine. There's no external entity that can withhold the keys needed to access your data. Further, the data format of files saved by FOSS software is by definition itself openly available. The code is there and available, to be extracted and used in whatever way you need to be able to get to your data.
FOSS is about keeping control and choice in your hands. How important is your data, and access to the software to manipulate it? I'd wager the answer is "Very Important". In that case, can you take the risk of using non-free software?