First paragraph you have have literally just proven they are versatile and capable in filling all three roles. We don't have the full kit (or we do) who is to say they won't expand more into it, not just that but Aetherial Mimicry reinforces this especially when it buffs certain spells related to certain Pseudo roles. Regarding for roles, it can involve two ways. Either way I don't mind DPS only or all 3 roles. However it is better keep people happy if it can do all 3 roles which would be the much preferred outcome. White Mage did have Stoneskin and is not a tank, however you cannot apply that to a Blue Mage in FFXIV. Especially when some of the spells you obtain can be boosted with the role. Pom cure is one such example that the normal healing potency is 100, but with Aetherial Mimicry: Heal the potency jump to 500 as strong as a normal cure. If you want a directly similar spell then lets take Cactgaurd as another example where the damage taken is reduced by 5% and only other party members can receive it when Blue Mage has Aetherial Mimicry: Tank it changes to 15%, it is a type of mitigation but it is only strong when a Blue Mage's pseudo-role is tank. It does sound similar to another move from another job, which is Paladin's Intervention, it does the same thing except with 10% reduction in damage taken. If you want to discuss FFXI, everything has the potential to be just about anything so long you have the gear set. Blue Mage takes it a step up and includes spell set as well. Each spells have a different trait that can potentially change Blue Mage's Stat and sometimes with the right spells you can gain a new trait. For example there is a Blue Build that lets you nuke mobs quickly and easily as a group, but you can't perform it unless you have 2 gearset, 1 macro (With the spell you want) and the specific spells that grants you the traits Dual Wield I, Fast Cast 1/2, Magic Attack Bonus I-IV, Defense Bonus, Magic Accuracy Bonus, Magic Evasion Bonus and Conserve MP. This setup will let you solo nuke a group of mobs regardless of level (except it wont work in instances so you need a different gear sets). Not just that but FFXI has sub jobs, meaning you can have one job at 99 and have the abilities of another job up to level 49. Literally the possibilities are endless with that (even though there are some recommended and build and some not) but regardless anything can tank, heal, support or DPS.
Both Red Mage and Blue Mage have the potential to multi role, however they made Red Mage a DPS only with a bit of utilities. I am ok with that. If Blue Mage were to multi role the possibility would be endless and maybe, just maybe we have a new type of role known as an all rounder. I mean G'raha Tia already counted as an All Rounder role, who knows maybe it can apply to Blue Mage when it becomes a normal job. That being said I am still ok with it being a normal DPS, but again I think people want it to multi-role. Why? I am not sure other than the fact that most of the western MMOs you can multi role with just about any class and emphasis on versatility.
I see Aetherial Mimicry as a form of a stance, like the tank stances we have currently and some job gauges like DRK, MNK, NIN, SAM, BRD, DNC, SCH and SMN have 2 different gauge each of them function differently except some hold stances/buffs SAM, SMN, MNK, SCH and DNC has a counter bar varying between 3 to 5 where they hold aetherflow, the type of Sen, meditation stack and the feathers. DRK, NIN, BRD both have a countdown bar as a second bar. Aetherial Mimicry can work like that alongside changing its role. If they want to go with either a Pure DPS or a Versatile route I am all for either as long we get rid of the Limited Job system as a whole.
Since you love to take druid for a spin I'll give you a few other job examples. The classes you get as a paladin differs depending on your class you pick. We have just about 3 classes allowing us to have 3 roles. Here is the thing, our kit differs from each other. Same goes with Druid, Monk. The kit changes every time you change your sub class. The same could be said with Aetherial Mimicry where certain spells gets buffed depending on the role you mimic. Even then in WoW the sub classes sometimes share the same abilities even when their kit is different.
As for how to pull it off with over 120+ spells it can be done. Make some spells useless, adjust the key ones, the gimmicky spells doesn't need to be change however future bosses must be immune to their effects regardless of where they are FATEs or Dungeons. As for making it into the game with the other iconic jobs, it can be done but the voices need to be louder. PF exclusive is never a good idea unless it is meant for something like Ultimate raids then I can see why. But overall it is not, it is not ideal compared to a duty finder system. One moment a slot could be open and you joined it, next thing you know sorry can't join slot is full. PF is meant to do organized events/raids and there is a lot of PF for raiding in savage and ultimate. Plus we live in an age where everyone has a job, you have to consider some people don't have time to commit to a group/static. Everyone has to understand that PF just works with certain people and sometimes doesn't work with certain people, it is a double edge sword.
I'll be happy when they remove the Limited Job Status off of Blue Mage and never repeat Limited Job ever again. Unless they are transparent and say Limited Job is a type of beta testing for a job. In which case I wouldn't mind it, if it was a type of a Beta Test system then I'll keep voicing my concerns over at the Official forums in both English and Japanese cause I want the game to get better and not set on having mediocre effort put in.