No its about sacrificing one thing for another. You shouldn't have good ranged ability as well as very similar melee ability as a pure melee guys. Thats not balanced. You have to take it on a case by case basis rather than blanket "keep all classes pure". Like for example shielders aren't necessarily unbalanced by allowing them to have a 1 hander as well as a spear. They are still fulfilling a similar role. A bow user can use 1 or 0 slot weapons which give them mediocre melee ability compared to pure melee guys (and tend to use weaker armour). A xbow user doesn't need to sacrifice much at all for his build, it is nearly pure melee and pure ranged so the devs need to limit their capabilities a bit to try to create balance.
Jack of all trades, master of everything doesn't work in games like this. You shouldn't have the best xbows and 2handers/poles with very similar builds/capabilities as pure melee guys
Ah, but Grumbs, you can have an archer/shielder with: 1 slot bow, 1 slot quiver, 1 slot sword, 1 slot shield. Afterall, according to you, 100 or 150 wpf doesn't change a thing.
Please note, I'm not disagreeing that crossbows need to be limited. I simply disagree with the means to limit them. If you were to attribute WPF checks to equipping the crossbow, versus a slot (ie keep them one slot) then you have limited their ability in the same fashion as you would other weaponry (since now they have to train in it).
There are already limitations in place otherwise. Standing very still to load a crossbow, plus the load times are far worse than an archer's. Couple that with the number of bolts carried...