Well true, but how do we get to instanced content?
Quests, or leveling through dungeons.
Obviously flying doesn't affect leveling through dungeons, but it does affect the rate at which we quest.
Now, if you take an early-Cata approach, where you had to actually reach the dungeon entrance before you could queue for it, and you tie unlocking dungeons into the quest progression, then by slowing our questing, they've slowed our leveling. They've slowed all pre-max-level content consumption. If it is taking longer for fewer people to reach max level, this will have a ripple effect of leaving fewer people capable of dungeon or raiding at max-level. Second to this: Blizzard has stated they want to increase "max level questing".
If we follow the model: all content is gated on the basis of quest completion, then by slowing our quest completion, they've now slowed all forms of content consumption, even at max level.
Which basically means, as was pointed out: less work and more profit for blizzard, while players are saddled with more work and less content.