I'm not a fan of the idea of specialization in this manner, as I don't like feeling like my character is 'incomplete'. The ability to freely switch jobs was one of the 3 major selling points that drew me to this game.

What I would be OK with are limited-duration 'guild allegiances' or something to that effect that allows you to use certain facilities or get access to certain materials from one guild for a period of time.
There are 2 real ways this could work:
1) You get credits for the time you're in the guild (and can potentially earn more through dailies):
Think of it kind of like leve-allowances that only stack for a particular guild for a period of time, only these are required for high-level crafts.
You can change allegiances at any time, but these then reset.
or
2)
There are certain, untradable crafting mats or facilities you can use and can buy freely, but only while you've pledged allegiance to a particular guild. You cannot change your allegiance once pledged for a specific period of time.

Basically, anything that permanently locks you into one path though is no good...