What efficiency? Completing quests faster? But you won't be able to complete more quests if you are flying, you will just do it faster. Aren't those who are against flying saying that they are against because they enjoy being on the ground? So, what's the issue? If what they say is true, they will complete the same number of quests in the manner they enjoy and those who like flying will complete them in the manner they enjoy.
No, it doesn't work like that, because those who are nominally against flying in reality frequently are ambivalent, and they are just too quick to buy into Blizzard propaganda regarding it, and too slow to actually think about it.