Personally I think it should teach slings to keep things consistent and balanced, even though it does not make a lot of sense to learn slings from parrying with a staff. While the HB exp from parrying with it is very minimal, and not really important, you shouldn't be able to learn two weapon skills from one weapon at the same time unless both skills are checked when using the weapon. The GMs have been doing a great job in getting systems balanced. I would love to see it applied to the insignificant things just for consistency.

And just in case someone throws out an argument about it checking the HB skill when you use it to parry, then you shouldn't have to train 2 different weapon skills to use one weapon effectively. Staff slings are made to be parried with, if weapon skill is still checked in the parry calcs then it should check the skill of the weapon you are using, not a weapon skill that has nothing to do with it.