Honestly, I'd be fine if my party played in a sub-optimal way if it was negligibly sub optimal in non-savage. For example, I tolerate it when my healers don't dps, even though when I heal, I dps and I think it's best. Jumping to "ice mage" tolerance is a bit extreme. I think what OP was suggesting is different ways to play/build a job that are all "right," some play a little better or worse depending on the context.
That said, I don't think it will ever happen because Yoshi has been outspokenly against that several times. He has concerns. I'd like to illustrate these with a (simplified) example:
Say on black mage there are three builds: fire, ice and thunder. Say that they somehow manage to balance them to where there's a negligible difference between each for most content, which in and of itself is a big assumption. And say you had to lock in your BLM to play a certain way because gear stats, or whatever. So you gear a "fire" mage. But when the next raiding tier comes out, ice mage is ever so slightly favored for the raid. So only ice mages get taken into the cutting-edge content. Sure, you can roll fire in the dungeons and normal mode trials, but if you got locked out of raiding for a cycle, that would suck for you. And even if the next raids people started taking fire, you're only in the raiding content for half a year before it gets switched again. Why stay subbed if you play for the raids you can't get into? It's not a good business model.
EDITING REPLY HERE BECAUSE THAT DARN DAILY LIMIT. Ahem...
My reply to this is that if it's so easy to switch between builds, then this is something we already have. For example, I have a different build for my SCH for PvE vs PvP. One is all MND all the way, the other stacks some fending accs to increase my health. Two builds, two gearsets, one job. This is what you're asking for, correct?
What is really needed, then, is not the capability, but rather the content. PvE and PvP are obvious examples of content that requires different things from its jobs by nature. As they say, Healers are Tanks in PvP. If the raids were designed to emphasize different sub-stats, for example, a boss that aggroed to the healers for a certain move that couldn't be survived without the added HP from fending, then we would see those builds come alive.
The difficulty with that and why it's hard to make such content is the limiting loot system. I greeded my fending accs and got them from luck. Imagine needing fending for a raid as a healer, when you're only allowed to greed and the tanks need them too. It's prohibitive. And tomes? If I needed MND for one turn of the raid, fending for another, aiming for another (think, high-accuracy cap boss), I'd be dropping weeks and weeks into the accessories alone. And some people have more than one job and more than one character: the gameplay time/grind required for this would be off-putting, I think. I could be wrong.