Model Two – Everyone has specialties and you match the spec to the situation
Under this model, we would establish spec specialties. For example, Arcane could be good for single-target fights while Fire is great at AE fights. Some of that design already exists in the game, but we try not to overdo it. If you really like playing one mage spec, or really detest constant spec swapping, then this model isn’t going to be to your liking. Furthermore, we don’t want to overstrain our boss design by having to meet a certain quota of AE vs. single target fights and movement vs. stationary fights and burn phase vs. longevity fights or whatever. It is also really hard to engineer these situations in Arenas or Battlegrounds (for example, both mobility and burst are extremely desirable in PvP), so in those scenarios there still may just be one acceptable spec.
The problem I see here is that WoW is way too limited in what DPS can bring to the table.
Since Ghostcrawler used a mage as this example, here's a mage (elementalist) from a Guild Wars perspective.
Water is good at defending the party, and shutting down both casters and melee (not very good at damage).
Fire is good at burning burning burning. Masses of things, or single things. BURN! Defence? What's that? Shutdown? Well there is about one spell, otherwise, the best shutdown and defence are BURNNNNN THEM ALL.
Air is good at single target spiking (conceptually, the best single target ele damage), shutting down physicals, and to some degree, casters.
Earth is good at defending the party (though not as good as water), but does a decent amount of aoe damage (not as much as fire), and an equally decent amount of shutdown. Not so good at single target.
*pokes simplified GW elementalists* *ponders WoW mages*
...poor buggers really don't have much of a choice at all.