I personally feel this would be a nice and interesting addition. Depending on what the masteries were, you might want to seek out someone who has mastered the desired craft instead of just going to someone who took every single craft up -- and you would need to seek someone else out instead of just doing everything yourself, which I like. Everyone seems to tote around this "It is a MMO so you should talk to and do things with other people" idea, but every time people see SE doing something which would facilitate that idea, it gets squashed down, usually by the same people who were toting around the whole community thing I just spoke of.
I think some crafters feel like they have really no identity since everyone can level every craft, they might find it hard to find work in this economy where everyone is the same, this would make them more unique and useful in my opinion. Everyone will have the same opportunity to master a crafting class, just like everyone has the same opportunity to level all classes etc.
Those are just my ideas and thoughts.