While I do agree that it would be nice to be able to have all the crafting skills one your toon at master level, at the same time, limiting the number you can master to one encourages community involvement in the crafting process. I've made a quite a few friends in game that I otherwise would not have met because I responded to a request to process some high-level materials. I'm not saying the one-craft-mastery-per-character is a perfect concept; I'm just saying there is a positive side to it.