I think the logic might be:

It's a certainty that your clients and colleagues will be using Word to work with you and the rest of the legal system, and some files will use docx.

It would be pretty bizarre to turn down work or make things harder for people outside your firm because you refuse to use Microsoft Word.

You could use another tool that handles .docx but if it's not directly compatible with Word you're almost guaranteed to run into compatibility issues or maybe serious errors. If you don't even touch Word at all, then the first person to run into them would might be your client. Do you not just immediately decide that it's not worth saving $100/yr to eliminate that kind of risk? If not then you have to say something like

  "Actually, it's more important that software be free and open source than that thing you pay me to do for you, that costs about two minutes of billable hours, and which every other lawyer uses?"

I don't think someone with that kind of decision making should be a lawyer, not necessarily because it's wrong but because even if it's true, imagine explaining to the bar association that Arch started opting you into nightly builds?

Obviously there should be lawyers trying to use open source software, but I can't believe a real person would never run into compatibility issues. Richard stallman as a lawyer explaining that he can't open files sent in formats that do not respect user freedom is such a good bit