This post at Blessing of Kings makes a simple argument: BC raids are too healer-intensive. The standard Karazhan composition is three healers, two tanks, and five DPS, which makes 30% healers. The typical 5-man, on the other hand, is one healer, one tank, and three DPS, or 20% healers. Basically that means healers are suddenly more in demand once raiding starts, and that's a problem, because there's always a healer shortage. BoK argues that this composition requirement -- 30% healers -- extends into larger BC raids, and that encounters should be made less healing-intensive because healers are hard to find. What do you think?
As a side note, it's interesting to look at heroics with regard to group composition. I would argue that heroics, like raids, work best with 30% of the group being healers, which translates into one full-time healer and one off-healer (ideally a shaman, in my opinion). This agrees with my overall impression that heroics are more like five-man raids than anything else.