You are correct for all of the above and that's why I do almost all my coding right in JMP. I manage over a dozen addin's that I've created over time and I recently set up an SVN repository to make it easier for me to track all of the addin scripts and documents.
I use Sublime Text as my editor and it allows me to edit the .def, .jmpcust, and .jsl files all in one location (not to mention a few other text-based files I have in the repository). I am familiar enough with many of my scripts that I can do a fair amount of editing outside of JMP using the program for final verifications of the scripts. As such, having proper coloring would be beneficial.
So then I can edit the .jsl, the .def and .jmpcust files all at once w/o having to jump between programs.