First off, LAN is a much better airline than the alternatives serving argentina, chile, peru, and ecuador. But there are a couple big downsides. First off they make you buy a foreigner fare, which is a lot more than residents. And they actually CHECK to see if you're a foreigner. Secondly, you don't earn many miles, because within south america most fare classes earn %25 or %33.

With aerolineas argentinas and other domestic airlines within argentina, you can buy a residents ticket, which are much cheaper, and they DO NOT CHECK to make sure you have a DNI (argentine residents card).

I make a point of trying to fly Gol within south america, because i earn miles and it's a decent airline. But beyond that, LAN's only benefit to me as EXP / OW Emerald, is lounge access.