Why don't they make all teleports free? Because people would bitch about it. They'd say "OMG you're making it too EZ mode!". Also they likely want to keep a restriction on it so people will actually use other means of transportation. Why walk when you can teleport? Why ride a chocobo when you can teleport? Why ride the airship when you can teleport?

It worked alright in Guild Wars but only because it was like that from the beginning (as far as I know) and the maps were packed to the brim with enemies so having to navigate them to get anywhere every time would get tedious and boring quite fast.