I'm thinking it is possible to have a Support class without breaking the balance of the game if the class was able to switch between stances/forms.
For example ...
RDM - could have Attack/Magic/Support Stances where each are enchanced when switched to but the others become limited.
For instance, the RDM switched to Attack stance it's attack power and sword skills become available at the cost of magic/support abilities.
If it switched to Support it would be able to use a host of significant buff/debuff abilities/spells at the cost of physical attack power and limits the access to higher tier healing/attack spells.