The second problem with flameglow right now is that it's too good versus teams with consistant pressure and not good enough versus burst damage (take that with a grain of salt).
You'd be surprised how much of the damage you take is below the Flameglow threshold. First, almost every spec in the game has at least 1 if not 2 DoT effects that deal 15-30% of their damage. Arms Warriors, UA locks, DKs, Spriests etc. DoTs are pretty obvious so no need to go crazy explaining. Auto attacks have gone back to what they were in BC and are accounting for a sizable portion of all melee class damage. These attacks pretty much never break the threshold. Next we have mastery effects. This isn't considered often, but most classes with damaging mastery effects have one that does small amounts of damage over time. Combat, Elemental, Arms, Frost, and Retribution mastery all do small amounts of damage that add up to 10-20% of their damage over time (except Frost because Icicles are bad). It doesn't matter if Flameglow isn't that effective against burst when a majority of the damage you take is small. There's really not a huge number of bursty abilities left either. Obviously nobody wants to play with the talent and it's boring and skill-less, but it's definitely worth about 20% passive damage reduction which is something Mages have never needed to survive.