Raids in Wrath of the Lich King need to be balanced around 20% healers. There are many reasons for this:
1. The introduction of Deathknights as a DPS/Tanking class means that there is extra competition for the non-healing slots.
2. Blizzard has done a really good job of making non-healing specs raid-viable, and that means even more competition for the non-healing slots. Let's say you want to bring an Enhancement and an Elemental Shaman. You need at least 2 Resto Shamans to fill out the healing quota, so you're looking at 4 shamans in total. That means you are taking one slot away from a pure DPS class, and that's unfair to them.
3. The rest of the game is balanced around 20% healers, so bring raiding in line makes balancing the population easier.
4. Healers have always been the hardest role to recruit for. Requiring less healers and more DPS will make more players happy.
Take a look at this proposed WotLK raid makeup:
Group 1 - Prot Warrior, Prot Paladin, Feral Druid, Deathknight, Deathknight
Group 2 - Rogue, Rogue, DPS Warrior, Enhance Shaman, Ret Paladin
Group 3 - Balance Druid, Mage, Mage, Elemental Shaman, Shadow Priest
Group 4 - Warlock, Warlock, Warlock/Mage, Survival Hunter, Hunter
Group 5 - Disc Priest, Holy Priest, Holy Paladin, Resto Shaman, Resto Druid
To me, that looks like a gorgeous raid makeup. Every class has 2 or 3 slots. No class takes more than its share, and no class is completely left out. There's lots of different specs, and lots of synergy and options available.
However, this setup is completely useless under today's raiding conditions. There simply aren't enough healers. But if raids are balanced around 20% healing, raid compositions like this can be viable, and I think that would make raiding healthier and more enjoyable.