My Mesmer is very supportive. He's always saying things like "You can do it", or "Good job Whammo"
I would consider a support role to be one in which the character is not actively seeking to damage the opposition.
I thinking labeling any class as purely supportive is a mistake. Most people would consider Monks to be a support class, but if a Monk configures for Smiting, he's not a support class. Elementalists are normally considered damage dealers, but an Ele would specializes in Wards would be a support character.
Likewise with a Mesmer. A Mesmer could be configured for a support role, but it is also possible to configure a Mesmer as a damage dealer, maybe not as effective of one as other classes, but still a damage dealer. Illusionary Weapon Mesmers don't provide much support, Interrupters and Hex/Enchantment removers do.
Almost any profession can be configured for support.