oookey, so here we have the average "strength" of the class/spec from patch 4.0 up until today:
http://raidbots.com/dpsbot/Spec_Scor...4/730/default/
Obviously the numbers on the right represent the current situation so it's pretty hard to get a read here.
This is ofc affected by the problem of "if one spec is weaker then most people will play the other spec, even if they're pretty close".
Looking at this I can pretty much tell that mages have been in a REALLY good spot for a while, having a middle of the pack spec and an absolute top one in every single tier (fire 4.0, arcane 4.1 and 4.2, fire 4.3 and 5.0 and now arcane.
You mention hybrid viability during the burning crusade and I recall a podcast from the time (the four horsemen with TB and a bunch of raiders from method, nihilum, death & taxes and a couple of other guilds) discussing hybrids. I think I remember them saying that shadow priests was an absolutely essential part of the raid (that includes their dps, not only the battery). So the notion that hybrids were useless is pretty harsh =\ You've also got the whole situation with shamans and Mu'ru.