Triggered by recent discussion here on the eternal "healer shortage" problem many MMORPGs have, Ben wrote me with an interesting proposal: If groups had 7 members instead of 5, there would be more spots for dps classes, and only 14% of players would need to play tanks, and another 14% healers. Problem solved? Not so fast, I'd say.
In my opinion the number of people interested in playing healers or tanks depends on many things, not just small group content. Consider a typical "career" in World of Warcraft, where you start out playing solo, then do 5-man dungeons, later 10-man raids, and maybe one day 25-man raids. The problem is making tanking and healing equally attractive in *all* of these situations. Blizzard already started that process by addressing the first issue, that damage mitigation and healing isn't very effective in solo gameplay, by giving every tank and healer a dps talent tree.
But when you consider the progress from 5 to 10 to 25 man content, you will see that the tank : healer : dps ratio isn't constant. In principle a simple tank and spank boss would just need 1 tank, even in 10- or 25-man raids. Thus Blizzard already had to give most bosses some special abilities which forces a 10-man raid to invite 2 tanks, like some debuff which forces a tank switch. But it is difficult to set up encounters in a way that they require 5 tanks. On the other side, a 10-man raid will often rather take 3 healers than 2, and a 25-man raid also needs more than 5 healers. Thus the bigger the group, the less players are needed as tanks, and the more players are needed as healers.
This is why I, as a casual raider, play a healer as my "raiding character". It isn't as if I wouldn't want to try tanking or dps in raids. But the increased demand for healers when going from solo to 5-man to raiding means there is always an open raid slot for a healer, and I don't feel as if I'm leeching, even if I don't raid on a hardcore schedule. My tank and dps don't have that advantage, and the tank is further hindered by the fact that tank performance more than that of other roles is gear-dependant. Thus a casual raid tank is a badly equipped raid tank, which makes him a bad tank.
So if we made small groups have 7 members, we would need to create 14-man raid groups in which the optimum composition was 2 tanks, 2 healers, and 10 dps. And 28-man raids in which 4 tanks and 4 healers were needed. So how do you design raid boss encounters like that? In a way that comes back to a previous proposition of mine, where I proposed that raid bosses should drop more loot when killed faster; if timing doesn't play a role, but survival is crucial, people will always tend to bring more healers.