I think Hachoo stated this well. Multiboxing multiple classes is only for certain people. Look around on the forums; you'll notice that only a few of us have hybrid composotions, and most of us who have mixed groups (more than 2 classes in our group) do it with 10 characters. Having all of one class is great for learning, fun, and "instant" gratification. You press 1 key, you get the same result 5 times. It makes macroing a lot nicer, and it's a great way to experiment with multiboxing. For someone new to multiboxing, I would strongly advise them to stay away from a giant mixed group; mainly because there's so much synergy required and I can't even count the number of hours (I'd guess around 30-40) I spent setting everything up, learning how to work with my characters, toying with agro, casting, and forcing them to work together as a team.

The original poster raises a good point though, and that's something that has been discussed here before - the viability of an all shaman team at level 80. It's mostly speculation now, but we are seeing tanks that are averaging 20k hps with blue gear. What this means is that we're going to see a shift in the way instances are dealt with, especially given the fact that Blizzard seems to be re-emphasizing the role of a tank especially in WoTLK. Will a 5 shaman team be viable? I don't know; from the looks of things I'd lean closer to saying no, rather than yes, only because warrior NPC types are surely going to be buffed up. I would imagine that 5 shamans could do instances with their burst dps, but I still think it may be a LOT more effective if a 5 shaman team put a tank class in the mix. Only time will tell if our theories hold true though.

Personally, I picked my 10 characters (4 of which are shaman, 2 pallys, 2 mages, 1 priest, 1 druid), to help support each other - especially in WoTLK. I could run ten shaman, but I don't think that would be very useful outside of pvp - in my opinion, there are major gains that can be accomplished by adding other classes into your group. Having mixed classes is not for everyone though, it can feel like more of a job (especially until you get familiar with it), than a fun experiment, but if you can endure it, the payout is sweet.

Cheers!