What I mean by this is that when you learn, say how to swing an Axe - you become rather muscular. You lose the delicacy of movement you need to have in order to make jewelry. Hence by choosing axe as a skill you are prevented from choosing jewelcraft also. Basically the idea is to make sure you don't have tank-flavor of the months running around, or the mule that does every kind of craft. It is more important that the skill selection restrictions promote variety in the worlds characters than in making sense.
Game play is paramount.
The role of a Crafter is to bring life to the towns - to make the town come alive for the rest of the characters. They are the people you come to see when you need a new sword, the ones who greet you by name even though they've not seen you for weeks. The ones who ask how the latest hunting trip went and whether or not you caught that elusive googolpik yet. No town is a real town without Crafters. There are basic principles you need to have in place for any item produced by a crafting skill.