Think of it the same as you would if you were hand levelling a toon. You progress much faster through levels as you are flying/not aggroing left right and centre through ground travel. You of course also fly straight to the quest source.. no having to go around/through that pesky mountain etc etc...
I am also of the opinion (I stress my own opinion) that you draw less attention to yurself when levelling/grinding and spending at least some of that time flying. I myself have a life membership and do not regret the purchase for obvious reasons..
Cheers.
