Proof-of-work is proof of work done.  Unlike node count, it cannot be spoofed.

Miners (a) attempt to earn a profit with their hash power, and (b) express their acceptance of protocol rules by way they construct blocks and by the blocks upon which they mine.  When a miner points his hash power at a pool, he forfeits (b).  When a miner points his hash power at NiceHash (the hash renting service), he sells (b) to the highest bidder (who right now are people who want BIP101 blocks mined).  

This is an example of a market becoming more efficient.  If NiceHash didn't exists, some of those same people would eventually purchase physical hash power, resulting (eventually) in a similar outcome (https://en.wikipedia.org/wiki/Coase_theorem).