I think the argument is that a lack of any value inhibits (or warps) emergent behavior. When money is completely free, people do things they wouldn't otherwise

I think the argument is that a lack of any value inhibits (or warps) emergent behavior. When money is completely free, people do things they wouldn't otherwise do. If you're trying to see whether a contract has a bug, you might want to try it on a real testnet. If you want to see how game theory plays out, a cheaper ethereum net (where everyone still has some skin in the game) is more interesting.