There's a recent, interesting blog post regarding tanks, with a rather solid suggestion on modifying how tanks get hit: A boss attacks only tank-spec players, and each hit gives a "-10% chance to be target of next attack" debuff that last for a few seconds (maybe for 10 seconds, if a boss attacks every two seconds).
A single tank would never take more than ten blows in a row (since he would have -100% chance to be target of next attack). This would fit well with the "healers need to worry about mana" paradigm, as the issue in WotLK healing was that healers have to land huge heals constantly (thus burning through mana) because the tank would go down in a one hit. As a player takes more hits in a row, a healer has to use their fast, expensive, and large heals (plus cooldowns), but could be assured that the more one tank was hit, the more mana efficient the healer could be, as they had a continually reduced chance to be hit again. A tank who gets hit twice in a row would need a fast, mana intensive heal, but a tank who has been hit 10 times in a row could be healed up using extremely efficient heals, as for the next 10 seconds he would not have to worry about taking a direct hit.
Adding an additional tank to this equation would make it even less likely for a single tank to get hit multiple times in a row. Observe: Having one tank means that tank gets hit 100% chance on each hit. Having two tanks, 50% chance each. With five tanks, each tank has an innate 20% chance to be the target of a given blow. The tank who gets hit first then has only 18% chance to be hit, if he randomly gets hit again, 16%. After five blows, he has only 10% chance to be hit while other tanks have 22.5% chance to be hit each (90% split between the four debuff-free tanks). Adding tanks allows for the reduction of healers in the raid, as the current healers can rely on using more efficient heals, stretching their mana further, knowing there's a continuously reduced chance of a single tank getting brutally smote five times in a row.
From the math in the last paragraph, you can note that each additional tank has a lesser benefit than the one before it, thereby leading to an equilibrium of healers-to-tanks, where it's better to have another healer than another tank.