I think your question is a good one. But I think it depends upon what type of shop you are trying to get on with, how interested are you in learning new things, do you run manual or cnc equipment, are you responsible for setups, and do you do any programming or do you hope to, and last of all, are you responsible for inspecting the parts?
From what I have seen, GD&T is not used very widely. I think the biggest reason is that a lot of the people making drawings don't understand it so they do not dimension that way. Probably the larger the company, the more likely they are to use GD&T.
I think getting some knowledge of GD&T is important. My thoughts are that if you can't read the print, you are basically guessing at how to make the part, and you also can't tell if what you made is correct. Basically the prints are the designers way of attempting to communicate with the machinist. If you can't read them, it is a problem.
I do my own designs and machining. If I had something critical, and needed to sub it out, i would probably use GD&T. It is great way to communicate in much more detail than with regular dimensioning.