Way back in 2005, in my third post ever written, I predicted that a 10-man would need 2 tanks, 5 dps, and 3 healers. And lo and behold, this prediction came true in TBC. The standard Karazhan make-up is 2 tanks/5 dps/3 healers.
However, my prediction should not have come true! It was based on the assumption that an WoW 1.0 ideal 5-man group was 1 tank/2 dps/1 healer/1 "fifth man" (paladin/shaman), and extrapolated that assumption out to 10-mans. In contrast, a TBC 5-man is 1 tank/3 dps/1 healer.
If we extrapolate that out, a 10-man should be 2/6/2, and a 25-man should be 5/15/5. Immediately we can see that those healer numbers are simply too low. When learning Karazhan you really need 3 healers. It's at the edge of possible with 2, but 3 is ideal. For 25-mans, you need 7-8 healers. My guild tried Gruul with 5 healers last week, and we just couldn't keep the tanks up. A shadow priest respecced and we got him down.
Where are these extra healers supposed to come from? Looking at recruiting posts, everyone is looking for healers.
The 5-man game assumes 20% of the playerbase are healers. The raiding game assumes 30% of the playerbase are healers. Right away we see a problem.
I think the raid game needs to be balanced for raids with fewer healers. It makes no sense to require proportionally more healers than the previous phase of the game needed. A raid that required 5/15/5 would be far easier to put together, and could be created from five 5-man groups.