I'm not saying that there should be hybrid classes OR pure classes, though I do think jobs should be "pure" that's what specialization means, I am saying that if a class like MNK is going to really specialize in DDing then it needs to outshine other jobs to a point where it's preferable to the alternatives, maybe not preferable but equal.
The problem that will always exist is:
If I can bring a WAR who can tank equal to a PLD and while doing that can DPS 3/4 as well as a DRG, and I can bring a host of BLM that can out DPS DRGs or MNKs, then why would I ever bring those other jobs. In a world of success=reward&efficiency=number_of_rewards noone will opt for the "slower" "weaker" "more difficult" except to get bragging rights, "We did this w/ a all jobs setup." and what have you.
So as it stands SE is working towards designing content that makes jobs undesirable, bosses and NMs with some form of hit from behind mechanic to stop MNK and DRG from being able to achieve the highest possible DPS for instance; and making some jobs far better choices in most of that content than the alternatives, BLM for DPS, ARC to avoid mob centered AoE, etc.
I couldn't care less if a WHM tanks Ifrit, someone put in the time/energy/gil to make that happen good on. I do care if everything ends up how it is now. New players get the, "You need to have BLM WHM WAR or your really not that useful" bit, everyone should be able to play how they want to on what class/job they want and still be considered useful.
Yes the whole LS events can take w/e you want, it's not the point, even in LS events a DRG/MNK/PLD is not optimal, yeah you can bring it, but your going to slow things down, make things more work than they have to be. So even in LS your going to get the same effect, just not so "forced" only "nudged" to more often than not go on "good jobs".
I head enough of this in XI, 20 jobs only 8 of which are commonly used for EG content, 4 are almost strictly for soloing, the rest are kind of "eh". That's not what we should "let" happen to this game, it sucked in XI when ppl said what do you have and I said "string of jobs" it was ALWAYS nin/war thf/nin war/nin|sam mnk/nin mnk/war or blm/rdm it got old, I quit playing because of what we see taking place here already, jobs are outshining others to a point where people will eventually only level them for the sake of saying, "Yeah I've leveled everything.".