Extreme specialization

My mage in World of Warcraft is an exercise in extreme specialization. He is specialized in frost damage, and nothing else. I hit level 50 during the weekend, which meant that he can now use Flasks of Supreme Power, giving +70 to spell damage. Expensive, but it's okay, my warrior can make them and gather the mats. So with the flask, the wizard oil, and all of the "of the frozen wrath" armor my mage is wearing, he now has over +350 to frost spell damage. My frostbolts do over 700 points of damage, nearly 1500 for crits, and as mobs of my level have less than 3000 health, I can kill them with 2 to 4 bolts. They rarely ever reach me.

I have an addon named Recount, which measures damage per second. Funnily my level 50 mage does more dps on a level 50 mob than my level 70 protection warrior does on a level 70 mob. Because the warrior is also specialized, but in tanking. That works well in groups, but not all that well in soloing and PvP. Soloing with the mage is very easy, I'd guess he'd be very useful in a group, and in PvP as well.

Why is it that of the three group roles: tanking, healing, and damage dealing, only the damage dealing is really good in soloing and PvP. The other two abilities seem like support roles, and when there is nothing to support (solo) or very little cooperation (WoW PvP), these characters aren't quite as good. No wonder tanks and healers are always in short supply for groups, people prefer classes that solo better. Is the future hybrid classes like druid healers and death knight tanks, who can then easily switch to damage dealing when soloing? Or will we see better damage abilities for holy priests and protection warriors in the future?