lore.
plus, this isn't WoW. it's TES, which has its own style, separate still from other games.
I LOVE when people on these forums think riding dragons is "against" the lore and they themselves don't know it.
Riding dragons is NOT against TES lore, it's in fact part of it. There are accounts of dragons helping people and also of a few of people riding them.
Dragons are intelligent beings, some are good, some are bad, etc. It is not "far fetched" or out of the world for a dragon to allow someone to ride them depending on the situation and who they are.
As far as balancing goes and thinking it be "overpowered" umm....no, it's something you could easily balance. Remember, you wouldn't be the only dragon in the sky. Look at a game like Drakan, one of the few games where you ride around on a dragon (and dismount/fight on foot whenever you want). It worked fine in that and was balanced.