Yeah true enough I can see why Pashtuns would want to join Afghanistan and get rid of the Durand Line, though its too bad for Pakistan should it happen.
Fact is you can’t stop somebody from what they want to do and eventually you should let go
That said the whole plan has a few kinks, Pasthuns should realize that Hindko speaking people live in NWFP as well, and we would never join Afghanistan so that is something that would have to be resolved (though you could always split Hazara disctrict from NWFP).
Also I would guess there are more Pashtuns in Pakistani areas like Karachi, Lahore and Islamabad then there are in NWFP, that would kind of defeat the purpose of seperation. So that issue would also have to be resolved.
But that said if Pasthuns really want to join Afghanistan they should be allowed to.