Actively choosing closed technology even when free software is available always gives up certain abilities and/or rights, and creates situations where it's easier for small groups to maintain control over the masses.
Ideals that present the logic of being able to choose both between free software and proprietary as a workable or even the best situation forget the basis of closed technologies - that they aren't just personal secrets, but are often backed by law and seek to enforce that closed nature onto other companies and end users, including hindering the creation of free software to provide or inter-operate with them. By doing so, they seek to negate certain things that are seen as important abilities and even users rights, inherently making the combination of the 2 ecosystems incompatible unless you accept proprietary software as not any kind of problem for users, developers and technology development as a whole.
Being able to choose between open technologies and free software and proprietary software is barely even a choice if you want to maintain a completely free and open market for software and ideas.
Choice isn't just about how many players work with x codec, it's about allowing the creation and maintenance of choice in situations where it hasn't previously existed or is becoming limited, in which free software and open technologies inherently encourage. Proprietary software doesn't - it helps limit and reinforce the decline in choice, and helps further the creation of new monopolies where neither businesses both new and old nor users are able to break it - certainly not within a reasonable time frame to not be harmful to both.