Let's discuss Red Mage

[AlexionSkylark]

no they aren't, in that case mnk is a hybrid tank.
they just have some basic DPS, so they won't be useless, solo.
would you use a sch as a DPS instead of a DPS? would you use a sub-healer-rdm, instead of a real healer?

Erm... WHM and SCH are a capable healer with sub-DPS capacity (Id say they ammount to something between 1/2 and 2/3 of a DPS). That's what I mean.
And yes, lots of people would take a sub-healer RDM if the other healer is strong and well equipped, if said sub-healer would be able to pump up significant DPS.

Redmage could work as a tank pretty easily. Different elemental stances than have various effects. For example fire stance you get Enfire and Fire Spikes. Enfire adds fire dmg and increase threat generation while Fire Spikes does a decent amount of damage to those that hit the mage. Water stance's Enwater could be for replenishing mp/tp while the spikes reflect less dmg but increase parry. Wind stance could be the dps stance with the En increasing attack speed and reduce threat while the spikes increase evade. Then you have other attacks that change based on what elemental stance you are in. Like Elemental Assault, if you are in fire stance you do a cone attack for decent threat but in earth stance you do maybe a stunning aoe centered on yourself.

Add in some self-healing capabilities (I'd say wind stance could be the mp regen stance (RDMs wouldn't use TP imho) and water could be healing stance - fire already covers DPS neatly for soloing) and you got yourself a very original, and functional tank, too, who's very much in line with the RDM main concept of it primarily being a WHM/BLM hybrid.