That is fair enough of a reasoning if the spells relative ember generation pales to its other means in a fight, though it does also mean that if it is that insignificant it is usually a mastery stacking fight anyway for CB/SB/FnB reasons alone, and if generally speaking without such a case, mastery would be blatantly OP stat if it directly competed with crit or haste on ember generators
Afterall, you do not choose your main stat by 1 spell alone
I could also go into a whole discussion on stat synergies but I decided to simply keep the discussion focused on the gear available to us right now where it doesn't yet get too crazy