And what of transactions out side of your "economic cluster"? Or is that just shorthand for saying: Within this cluster of nodes, even though each of us is only watching a fraction of the network, we are collectively watching the entire network and will inform each other if we see anything relevant to other nodes.
People don't care about such transactions. If Alice doesn't interact with Kevin then she doesn't care if his transactions are legit or doublespendings or exist at all.
EDIT: By the way, if Satoshi didn't add the requirement to validate all transactions in a block would Bitcoin get the same scalability potential?..