There are only 3 reasons I've ever seen anyone use part msgs:
1.) To curse and or yell at someone in the channel without having to listen to their comments.
2.) To say goodbye
3.) To spam
#1 your suggestion wouldn't address because you most likely won't be cursing everytime you part a channel, just when you get mad at someone. #2 imho is a useless reason for it, why not just say "goodbye" before typing /part? The only one I see this feature promoting is #3, and I don't think anyone wants that.
