There is a balancing issue there. If this only applies to weapons and you prefer to play a wizard type then you might want to kil monsters with spells instead of weapons. Hence you don't use your weapon less frequently then a warrior does and your weapon is not so efficient.
In the skill point system now. All you have to do is add a skillpoint or two into your favorite weapon and it's effectiveness rises.
Of course in the example above it can be extended in such a way that even the effectiveness of spells increases with use, then the wizard doesn't need his weapon actually, his spells are already becoming more powerfull.
But it might also render having levels of most skills as being useless. After you have the skill and use it it gets better already. Having a next level of a skill would have to add a completely different effect than it does now, afterall why bother adding an extra skillpoint to it if you can train it yourself.
