Is there any cons of making your employees use open-source programs in your company?

I am planning to start a bussiness and I wonder why companies usually work with proprietary software, as Microsoft Word to quote the most famous one. Why do not they use Open Office (or Libre Office) etc.?

From my point of view, you can save a lot of money and help the open-source community by, for instance, giving them part of your benefits in form of donations.

I do not know any (medium-big) company that does this. Probably you could give me some examples, just to prove that this model of open-source usage/collaboration works rocks.