
It all just depends on how many people within your server group are queuing for dungeons within your current level range.
Of course the short wait time for healers and tanks may pressure some players to queue as a healer or tank just so they don't have to wait, but it is very impolite to choose these roles if you don't understand them. Make sure you know what being a tank or healer requires before you decide to try to queue as one.
The Rewards are Better
Other than the speedy leveling benefits, there are also other reasons dungeon leveling has caught on so quickly. Overall, you will earn more gold while dungeon leveling. Additionally, you will have more access to better loot. The loot that drops from the bosses in the dungeons is far superior to anything you'll find while out solo questing.
Don't Forget the Dungeon Quests!
You'll also be able to complete the dungeon quests while you're dungeon leveling. Dungeon quests are usually available in towns that are nearby the entrance to the dungeon. They must be completed while inside the dungeon, which means you'll not only be soaking up experience from all the kills you make inside the dungeon, but you'll also get to collect a massive amount of additional experience when you turn in the dungeon quests later. And if you didn't win the rolls on the exceptional gear that dropped while you were inside the dungeon, you'll be happy to know that the gear you earn as a reward for completing the dungeon quests is also better than anything you'll earn from a solo quest.