I think there's an edge case here where someone might have one tree that they're building to multiple objdirs from, and they don't want/need to clobber all of them. I would be very surprised if that's the case here.
I think bug 840571 essentially makes this a wontfix. If mach build starts clobbering automatically, and clobbering does so on all objdirs, it's just too dangerous. Besides bugs like bug 840568, there's really no reason for someone to have multiple objdirs unless they mean to, in which case they're quite likely *not* to want this to happen.
I'd rather not have "mach clobber" do this automatically. My reasoning is that I often maintain several objdirs per srcdir (for example, a debug objdir and an opt one), and when I do "mach clobber && mach build" I want it to clobber only the directory I'm about to build. Perhaps a separate "clobber-all" command would be useful for some people?
I agree this is a WONTFIX. There *may* be room for a related feature request to offer a `mach purge` or similar to blow out untracked files from the source directory. This could just be a simple wrapper around `hg purge` (if it's installed), `git clean`, etc.