The reason they destroy gil I the former of taxes, travel cost and gear repair is the economy does suffer from mass inflation.

You can't have a system where 10's of thousands of gil are being created everyday from questing leve and dungeon drops without ending up in a year or so with every player having 5-10 million gil and everything on the mb costing thousands and thousands. Then the 700 gil you get from a leve will be worthless and the economy is destroyed.

That's kinda what happened in ffxi and they had to implement really heavy taxes in year 3 of the game.