If you did that it should take FAR longer to level skills through practice because you'd have to allot the time you used to gain PP's
Not really, lets take mining as an example. In theory you should train it only from PP you did on mining. Everytime you dig somethink, you gain experience and you practice. Once you practice the given rank to max. You realize you have also enough PP for training next rank.
So if you erase PP from use, people wont start advancing faster. I think only newbies will rank up faster. Because at the very start, only the PP are problem, later this problem disappears.
I have around 2,000 PP myself