Apparently, there is a perceived lack of quality WoW mage bloggery. I can't really fix that personally, since WoW is one of four MMORPG's I'm currently playing, and I don't even spend all of my time on my mage anymore. However, I have had a bit of time to respec the mage yet again (currently arcane for PVP and random instances, frost for soloing dungeons from the previous expansion), and that got me thinking about niches.
History of a new toy
The big shiny toy for mages in the current expansion was Frostfire bolt. Mages traditionally throw balls of either frost, fire, or arcane energy. Blizzard decided to shake things up in the new expansion with the "Frostfire" ball, a snowball that someone light on fire. Frostfire bolts are simultaneously frost and fire spells, which meant that they would benefit from talent enhancements to BOTH schools of magic. This put the spell in an awkward position where it would either be too good, or not good enough.
At Wrath's launch, the scale had swung too far in the "too good" direction. Fire-based Frostfire specs delivered almost as much damage as pure Fires specs, while offering greatly improved mana efficiency and solo capacity. (Remember, dual specs did not arrive with Wrath's launch, so this was actually a non-zero issue for the pure fire spec in the days before gear inflation broke the difficulty curve.) No one used Frost-based Frostfire specs because Frost's damage was already low. This was a very good thing for mages in the longterm, because Frosty-fire delivered even more control than a pure Frost spec, and something besides impact would have gotten nerfed if it had been more popular.
Anyway, it didn't make sense for one spell to obsolete both of its parent specs, so changes were made. Pure Fire specs got better mana regeneration and more damage, rendering Fire-based Frostfire obsolete. Pure Frost got more damage and a new replenishment ability that specifies that it only works on frostbolt. Today, the main highlight of getting Frostfire Bolt at level 75 is that it turns your Brain Freeze fireball procs blue.
Failing to find a niche
I've actually spent some time with all six possible primary tree + off tree combinations during the Wrath era, and Frostfire specs always feel like they're competing with the pure elemental specs. Pure frost has a niche in controlling foes. Fire has a niche in raw damage. Arcane has a niche in speed and efficiency when burning through encounters quickly. There isn't really room for a Frostfire niche, and, indeed, it's not entirely clear what the developers intended for one.
This type of challenge is not at all unique to WoW - take, for example, EQ2's struggle to find niches for six tanking classes when raids don't need that many tanks (and certainly not the sub-optimal ones). It just raises an interesting challenge. Someone had a creative, if perhaps slightly unusual idea and turned it into a spell, but it doesn't seem to have anywhere to go from there. Maybe the spell will get reworked from a primary nuke into something more situational in the future. For now, though, it appears to be a solution to the non-existent problem of what to do when facing an enemy that is immune to your preferred element.