If you are a tank/XXX hybrid, you will almost always be tanking. You may struggle to find a spot since there are a lot of pure tanks. Shadowknights are one of the most played classes due to their fun factor, so pure tanks are usually aplenty.
If you are a pure healer or a heal/XXX hybrid, you will always be healing. You will never struggle to find a spot. Most healers also have buffs or debuffs, but, while nice, aren't really what get them into groups.
The dps is then broken down even more. These are ranked (roughly) by current ranking in endgame dps:
1) Top-tier Pure DPS (Wizard, Assassin)
2) Bottom-tier Pure DPS (Warlock, Berserker, Ranger, Necromancer, Conjurer)
3) Melee Buffer/Debuffer (Swashbuckler, Brigand)
Caster Buffer/Debuffer (Coercer, Illusionist)
4) Melee Buffer (Dirge)
Caster Buffer (Troubadour)
Some of the buffer/debuffer classes (I'm lookin at you bards!) are really hard to level. Their buffs and debuffs are really tops and they will always get into groups easily, but good luck getting quests done due to their low killing speed. Buffers and debuffers are usually sparse in raids, but that depends on if they are used more for their group buffs or if they are used for raid buffs. Obviously, depending on the patch, group 1 and 2 mix up some. This is the current state, however. These can also vary some based on group composition, too.
Edited, Apr 30th 2010 9:50am by Maarg