>The scheme neatly solves the free-rider problem that plagues other permissionless systems. FTFY A parallel example would be various P2P filesharing protocols

>The scheme neatly solves the free-rider problem that plagues other permissionless systems. FTFY A parallel example would be various P2P filesharing protocols which was plagued by freeloaders from day 1, and had to come up with various schemes to regulate the user's contribution. The simpler solutions are not exactly Byzantine fault tolerant (user profiles in ed2k) whereas the more elaborate mechanism typically requires a centralised record keeper(private trackers, etc). A lot of the inefficiencies of PoW are necessary to keep the network permissionless, and conversely you don't need PoW if the system has any kind of entry requirement.