I think that nerfs in general were needed, but how they've done it is terrible. I don't like playing fire because of RNG, and because in a 10man heroic raid setting, bad rng did not only screw you, but your whole raid. On the other hand, if you get the right crits (which happend more often with increasing gear, obviously), Mage dmg became uterly stupid. I remember Gara'jal HC firstkill, with the players in our raid averaging like 25-27mil boss dmg on a 6 min fight. When I had a good pull with bloodlust, I did like >10mil DMG in less than 1 min, which is 40% of average player DMG on a 6min fight. And then there were trys, where I had 5mil dmg done after 1min. Those trys, we wiped on the enrage. We finally killed the boss, when I did good pull DPS and nobody screwed up til the end.
For me, RNG is just bad design, even more in a small 10m raid setup, because your team can not rely on you, that you are capable of fullfilling your role.
Unfortunately, heroic firstkills are often very tight, and we mages have no other "heroic viable" specc. I still refer to 10er raidsize, because every player there has a lot more dutys/more movement than in 25m raidsize. In 25man, arcane is an option, because you can stand in the rune. In 10m, arcane is rather unplayable. An for frost, it can be viable on a few fights, but unfortunately, frost sucks on bosses with +dmg phases, because of the lack of burst.
So nerfing fire somehow makes mages even more redundant/less popular in 10m heroic progress raids than they are now. Of the Top20 10m guilds, only 6 Mages were found in the "firstkill-setup" of 20 raid groups, compared to classes like druid (29), priest(27), paladin(28), they are 5 times less popular. Most progress orientated 10m guilds rather pick a second druid/priest/pala for their team before filling it up with other classes, then there come warrior, locks, dks, hunters (about every top20 raid group has 1 of them), then they pick shaman (11), monk (10), rogue (10) and at last mages (6). And that was even before the nerf.
So I wonder, why other non-hybrid pure DPS classes (hunters, locks, rogues) have the edge over mages in 10m HC progress, despite mage being the best DPS class?
And the only answer to that is, that mage DPS is not reliable and beside DPS, mages bring nothing to the table.
Now they nerf mage dps (which is fine), but also make them even more RNG dependant, and still give them no utility at all.
So what they should have done, is nerf pyro spell coefficient by 40%, but make it less RNG dependant to get. If pyros hit less hard, overall dmg AND combustion dmg is lower. When combustion dmg is lower, you can remove the 3-target cap of combustion, which would Fire mage make once again a good AoE Specc, but a medium single target specc.