The way the system should work : The wpf requirements are in ADDITION to the current str/pd/pt requirements. This will keep all agi buffers from exploiting the system. Give a little boost to WM, but not too much. At 5 WM, you should be able to have 160 in one (1) category. 160 is a good number for the top tier weapons, like poleaxes, flamberges, sniper xbows, throwing lances, and sideswords. I'm sure we can fill an entire thread just discussing the weapon tiers, so I'll leave it there for now.
The reasoning behind the system : It'd be a boost for people that want to specialize in a weapon class, when there is currently no good reason too. It'd be a nerf for hybrids, who currently have no need to sacrifice anything for their builds. It would stop pure build melee players from exploiting the fact that you can still kill with a sniper xbow at 1 wpf. It would stop pure archers from pulling a poleaxe or flamberge out of their tails. If you are a melee player that is great at crossbowing, more power to you, but you're gonna be stuck with a bottom tier xbow. Add the wpf if you want to play a crossbowman. Archers, you may be really skilled at melee, but you are going to be stuck with the lower tier weapons. Sacrifice some of that archer wpf for a melee weapon if you want to use the good stuff.
Tweaks that will be needed (most likely). Un-nerf archery a bit. They don't need Pierce damage, but a bit higher cut damage at high wpf would help alleviate the pain of not being able to use the best melee weapons any longer. Decide what weapon goes into what tiers. You can probably divide them into 6 tiers, with a manageable number of wpf for each tier.
And the number one reason I'd love to see it : It works along the same lines as chadz's desire to make heirlooms special and meaningful. It'd be nice to see the top tier weapons in the hands of those who gave something up for them, rather than random joe-blows from all walks of life that came into some money.