From the Live Letter Q&A
If I interpret this, then Specialist is only a means for crafters to successfully craft in their specialty without the need to have all other crafts leveled for the cross actions. I'm hoping that what this also means is that if you choose not to specialize and just continue to cap all jobs to 60, you don't lose out on crafting effectiveness or access to recipes.From level 55 you’ll be able to undertake a quest to become a specialist, and you can do this for three classes of your choosing. It will be possible to change your specialist classes by using a new item known as "scrips." There will be certain actions that you can only use while you are a specialist, but even if you do not become a specialist, you’ll still be able to take all of the Disciples of the Hand classes to level 60.
While I said that the concept for crafting up until now was such that it’d be fine to just level up any one class, the development team is aware that actions like Byregot's Blessing are a necessity, and that leveling all classes is required to use higher difficulty recipes, making crafting quite difficult. The specialist system is something that balances this out, and allows players to enjoy crafting with more choices.
If all of the above is true, this could be a welcome change - as long as you don't NEED to specialize to access content.