At the end of the day, there are two possibilities for the consideration of the presence of various specs of the same class in raids or in any other aspect of the game.
First premise: All specs are viable for raid encounters, and so there should be some representation of all specs in a raid.
Now, if that "equal" or "appropriate" representation is not being met - and it may not be equal because you may only need one of a given spec for "utility" or other reasons - then the raid model is broken in that the utility of the other specs are not required. Perhaps the raid needs to make use of the utility of the spec that offers lower DPS so as to offer that playstyle some time to shine. This happened a lot in vanilla by the way.
Second premise: There's no need to consider that ALL possible specs of a given class SHOULD be represented in raids because there are other aspects of the game wherein the other specs have their "time to shine". I.e. PvP/Arena.
If there is no viability for frost mages in PvP over say, arcane or fire, AND there is no viability for frost mages in a raid, then there is a problem.
So the question with respect to mages is:
Is there a part of the game wherein frost mages are viable or even preferred? Do they make the best choice for an arena mage for instance? I don't know that answer, it's an honest question. If the answer is yes, then there is no problem and the rationale should be clear - frost is the PvP spec, and arcane or fire are the raid spec. Presumably there will be some raid encounters or raid dungeons where either fire or arcane would be preferred for instance.
All in all, the big thing that I think WoW has moved away from and which is the REAL problem here is the give and take, the necessity to make choices between things like maximum DPS and maximum survivability, or utility, etc.
To use an example from early BC - back then, max DPS for hunters was Beast Mastery, but that's not what got hunters through the rough heroics (and they were rough back in those early days - most of you probably don't remember that, but I do - back when those heroics were required to get keys to raid instances beyond Karazhan). Hunters would get invited to heroics because they could spec for utility and crowd control, not because they could do DPS.
This game has largely homogenized various aspects of class choices and I think it has done so, in the name of "simplicity" but it has done so, also to the detriment of the game.
-----signature-----
If ignorance were painful, half the posters here would be on morphine drips.
Everyone playing WoW knows everything about playing two classes: 1) their own and 2) Hunters