Bitcoin is money. Money is math. Math can be measured and quantified objectively. A trustless system is of the highest importance with a decentralized store of value. Proof of work/Consensus/Incentives are perfect for sound money. 

Nostr is identities and connections. Social connections are not math. They cannot be measured or quantified objectively. The fundamental principle of a social dynamic is one that revolves around trust. 

These are not the same.

Proof of work in bitcoin functions due to the dynamics between blocks, network consensus and incentives: IE the advent of blockchain.

PoW on nostr is just an arbitrary number to meet a relay's requirements. If a spammer has profit incentive, they simply scale their difficulty accordingly. Someone with an incentive will always be willing to pay a higher price (PoW) than a casual user. The obvious outcome here is an ever increasing difficulty that makes nostr unusable for the average user.

That said PoW notes are most likely part of the solution, but very unlikely to be the entire solution. If I thought it had no value II wouldn't have released notemine last week: https://sandwichfarm.github.io/notemine