yeah thats true. to be honest, i think skinners should be able to add them to the game granted only they would be able to see them. but it also would allow them to be like soundpacks people could download them and use them instead of waiting for the HTC team to approve them, it also may be better to increase the amount of skins that a player could have. then again HTC guys know what their doing so i leave it to them.
Joking aside, I believe it works something along this lines of this (someone correct me where I am wrong):
A RES file is simply a text file, containing a list of other files. The RES file goes along with your compiled skin files. A RES file is used to instruct the AH server to send the specified files to clients that do not have your custom skin(s).
As most of the objects in a plane remain the same (gauges, cockpit, etc) and only the "paint job" changes, HTC is the one to make the .RES files.
I think...