Ideally around 2 of each class should be in a raid. It can't be 2.5 because most raids require 7-8 healers and not 5. Changing this would be pretty difficult as it would mean adding stiffer enrage timers and other penalties for bringing too many healers. Shamans, paladins and druids (and maybe priests) are probably going to be closer to 3 per raid because their specs are so different and one of them can heal. The question remains of who goes home if the DK comes in? Though a just as valid question is who goes home if the moonkin comes.
First off, Ghostcrawler is probably right in that it is harder to create interesting encounters with fewer healers.
However, my problem is that it doesn't look like that many people want to heal. In my experience, healers are almost always the bottleneck in recruiting, and they burn out the fastest. Then the top tier guilds recruit healers from the lower tier guilds, and that means the lower tier guilds have even harder time picking up new healers.
At 8 healers, that means that each class uses 2 of their 3 spots on healers. 2 of 3 paladins are Holy, 2 of 3 shamans are Resto, etc. There's only one spot left for a non-healing spec. Want to be a Prot Paladin? Better hope there isn't a Ret paladin in the raid.
The basic fact is that the number of healers Blizzard wants to balance around simply does not match the number of players willing to heal. That contributes to the instability of raiding, and makes it harder for people to actually raid.
The thing is that a lot of people won't adjust to the healer requirements. If it's a choice between healing or not raiding, a lot of people will choose not raiding and end up quitting. Healing is simply "not fun" for many people, and trying to force people to do something that is not fun "for the good of the raid" will drive them out of the game.
Lack of healers causes guilds to fall off the path of raiding. If encounters were less interesting, yes, that would be a loss. But the last three years have shown the reality of healing, and Blizzard needs to balance around reality, not an unsustainable ideal.