A bit late due to vacation:
> To me, a command substitution that runs in the background seems weird > and like something to avoid. Is there really a case where this is useful, > or would it be reasonable to include a suggestion that it's best to avoid > such a construct? The case that led to the PR mentioned was a shell script forking a (shell) daemon process with the need to know that daemon's PID (to register the incarnation). So there was the fork-and-background chant followed by an "echo $!", and all that surrounded by something like daemon_pid=$(...). If you want to know even more detail, it was dotcache (http://www.math.uni-bonn.de/people/ef/dotcache) and I worked around it by invoking the record-pid code inside the backgrounding code. And to further complicate things, that code was written for Debian, so dash, and the whole thing here started just because I was investigating whether my problem was specific to dash or triggered on another ash as well.
