Lip service is all it really deserves. Look at it this way. The original set of spell masteries were written up and published in the core book because that is all they had thought of (and tested) at that time. The rest were invented later by different authors. What was needed was an excuse, however flimsy, for why characters already created and in game might not know these new masteries. And provided an easy way to gain them.
Fiction, Fiction, Fiction.
That is all it is. What fiction best fits your personal aesthetics.
I myself allow players access to most masteries with a tenuous excuse, if even. Really, I do not even care. Some GMs like to be restrictive. I take the opposite approach.
If interested, I had years ago created Spell Mastery reference and HRs for my saga. Have a look at them here. Most of the new ideas were introduced as an experiment, and most have never been used. If I were to revise the list< I would trim it doen and toss out many of the ones no one ever picks.