As others have said, if you're joining one of the top raid guilds then class need isn't a huge factor as they've already leveled up Cleric and other bots for people to play if needed. So the class you pick doesn't matter as much, although being somebody who doesn't need to play a bot because your class is helpful is a little bit of a plus but not hugely necessary.
As far as EXP grouping, Cleric will definitely be needed from like 40+ on since it's harder for Druids and Shamans to MH higher end EXP content. Enchanter is obviously another big one for the sweet mana regen, great CC< and awesome charmed NPC DPS.
One class that people haven't really mentioned yet is Bard. A good Bard is useful on multiple raid encounters and you're also a huge boon to any group if you know what you're doing (great pullers, you have Charm and mez songs, and your buff songs for twisting). Plus if you can't find a group and you get the hang of swarm kiting you can level really fast and get yourself into that raid guild.
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