Proprietary usually mean "not Open Source".

Not JeremyNT, but from my perspective, avoiding vendor lock-in is my number one reason to favor open source software. Avoiding vendor lock in is very important to me when I am evaluating options.

Just because your business model and my business model currently align, does not guarantee that they will always align. I don't want you to be stuck being my vendor if I am not a good customer for you- and I don't want to be stuck using you as my vendor if you are no longer able or willing to provide the product I need.