Can you remind me why I need to spam the network in order to become a validating node?

edit: it seems to me the long con still, applies - or at least the simple sybil majority; if it doesn't cost me anything to become a validating node, it is trivial for my one server to pretend to be 10,000 or more nodes and just sit there acquiring reputation until I have a majority whereupon I can wreak havok.