imo diversity is good, after a while you get bored with your team and its fun to go off and do stuff solo, and the more classes you can play with the funner the game is. I break the game down into 5 different "meta-classes":

tank
healer
ranged DPS (just hunter really, playstyle is quite different)
melee DPS
caster DPS

while there are significant differences between say a warrior tank and a druid tank, end of the day the playstyle is 90% similar. Just like holy priest, resto shaman, or resto druid. So if you can make a group that covers all 5 of these "meta-classes" you can enjoy the game much more when you are bored of multiboxing. Then it just becomes a matter of personal preference; some will like a Deathknight tank over a paladin tank, others will love a druid tank. Similar for warlock vs mage vs ele shaman vs boomkin, etc, etc.

Hybrids are great due to overlap -- a druid can be a healer, tank, or caster DPS; a paladin can be a tank, healer, or melee DPS, shaman can be healer, melee DPS, or caster DPS.