>> [Adding Arun Isaac to CC.  Their commit d8796851 is the first one to
>> drop -pkg.el, but without explanation.]
>
> Commit d8796851 was an attempt to not install too many unnecessary files
> and be closer to how MELPA packaged emacs packages. See
> https://lists.gnu.org/archive/html/guix-devel/2017-04/msg00274.html for
> the detailed discussion.

In other words, no particular thought was given to -pkg.el. It was
simply dropped along with many other files. So, if consensus is reached
that keeping -pkg.el is a good idea, there is no reason to not do that.

Cheers!