I could see how the inclusion of BTFS in FreeBSD might promote free software, and its inclusion would not be prohibited by any version of the GPL.

But how does the use of GPL code in a proprietary project, as would be the case in your consulting example, benefit free software? The whole point of the GPL, any version, is to make sure that derivative work is itself free software. It's not that GPL "sounds good on paper" and in reality, it's totally different.

GPLv3 on paper: You can't use it to make proprietary software. You can't use it to take away users' freedom.

GPLv3 in reality: See above.