Context matters here. Emacs uses bzr because it is an official Gnu project, but it's buggy and increasingly unmaintained, so there are reasons to switch. Jordi seems to have some personal thing against Git, while ESR favors Git but is bad at interacting with humans. It's standard mailing list politics, with technical considerations irrelevant; IMHO the two most likely outcomes are sticking with Bzr and moving to Git, depending on how much ESR manages to piss people off.
I think this is the exact right question - without context hg advocacy makes little sense.
And when I personally engage in mercurial advocacy it with an understanding that as you say, unless it's 500% better than git – which it is not – it will never displace git. However hg is every bit as good as git and most of my advocacy (as in this mail) is merely point this fact out and dispelling quite a lot of FUD, which seems to pop up in these discussions.
My goal is to see Mercurial continue in its position as the Mac (or Linux) of dvcs' to gits Windows. That is as an honest, respectable choice that you might, or might not make for valid reasons.
> Git is an Open Source project covered by the GNU General Public
License version 2 (some parts of it are under different licenses,
compatible with the GPLv2).
"This is a brief heads-up that the reason I’ve been blog silent lately is that I’m concentrating hard on a sprint with what I consider a large payoff: getting the Emacs project fully converted to git. In retrospect, choosing Bazaar as DVCS was a mistake that has presented unnecessary friction costs to a lot of contributors. RMS gets this and we’re moving.
I’m also talking with RMS about the possibility that it’s time to shoot Texinfo through the head and go with a more modern, Web-friendly master format. Oh, and time to abolish info entirely in favor of HTML. He’s not entirely convinced yet of this, but he’s listening."
He's also involved in freeing code-repositories from their prisons, and porting them to other versioning systems, with the Reposurgeon. See https://gitorious.org/reposurgeon/
"reposurgeon enables risky operations that version-control systems don’t want to let you do, such as (1) editing past comments and metadata, (2) excising commits, (3) coalescing commits, and (4) removing files and subtrees from repo history. The original motivation for reposurgeon was to clean up artifacts created by repository conversions. It also functions as a repository conversion tool. Supported VCSes include git, hg, bzr, and Subversion."
GPLv2-or-layer is effectively GPLv2 plus recipient-friendly features. It is NOT suitable for the intended purpose of GPLv3, which adds restrictions on the recipient (for the benefit of 2nd-order recipients of derivatives).
Mercurial could be 10% better than git, but unless it's 500% better, it will never displace it.