Remember when it was a pain to level the non-pure classes? Oh sure, warriors and shamans could hurt things a good bit, but we were no hunters or warlocks. Don't even get me started on priests. Shadow? Psh, still a miserable experience. Smite smite smite for so long; an up-to-date wand wasn't a convenience, it was the only thing between painful leveling and deleting the character.
Want an alt that can solo well? Get a DPS.
Want an alt that you can level in less than a year? Get a DPS.
It's all backwards now.
Hybrids have a lot more damage than before. Quests and instances give gear that helps them kill. Their specs aren't split into Healing and Bad. They can solo like pure DPS classes, maybe better if you consider the self-healing for less down time.
Here's where it gets really crazy: instances. How often have you seen "lf2m instance healer and tank?" That's a few DPS who got together to run an instance and now they're stuck. What do they do? If they were hybrids, they might just rewrite their message to "lfm instance." They can tank and heal. If you're willing to do it, it's easy to get into instances as a tank or healer. As DPS, much harder.
Without instances soloing can end up as a endless lonely grind. And that is the fate of the pure DPS. They are like the elemental shaman laughed out of a group and the fury warrior without a shield; they are the useless exiles.
My three 80s are all tanking classes. My next highest character is a shaman. Somewhere in the mid 70s is a warlock, exiled and alone on the Alliance. Next is a priest. After that a rogue (exception) and then a druid (he tanks whenever he can). Below these are the hunter and mage. The druid is moving up fast, boosted by excellent soloing and easy groups as a tank.
Remember when the easy classes to level were DPS, not hybrids?