Flame reave makes more sense than firebrand to be class locked. There's is almost 0 reason to use flame reave on anything other than a spellblade. However, firebrand at least has buffs that might be interesting on other masteries.
Interesting take, I think that's the same logic we used to pick the opposite actually. Because melee is just a firm theme for spellblade and hard not on the other two.