We want to move content out of /etc, and since there is a good place to put them in /usr/share/
All files that can sanely be moved out of /etc in bash-completion have already been moved; the remaining are there for backwards compatibility which will be pruned sometime. A bunch of _other_ packages still ship their completions in /etc/bash_completion.d though; assuming bash-completion >= 1.99 they can be moved to /usr/share/bash-completion/completions but in many cases the filenames will need changing as well (they need to match with the command(s) they install completions for).
Thanks, I was just going through the ones in /etc/bash-completions.d on my F19 box and opening a bug on each. Very happy to see the alternative directory.