More overall hashrate is better, even if the providers are deluded (or "charitable"  ). This is the crux of the proof of work security model. The bigger the network hashrate, the higher expense there is to take control of it. Simple. So we shouldn't be forcing a technical impediment that prevents otherwise useful and economic mining equipment from being a part of the network.

Also, if the p2pool codebase is to be useful and open source, multiple IP's must be able to connect to a single p2pool node. These factors will allow some form of centralised pools to continue despite any protocol changes to enforce p2pool only mining. Current p2pool allows people the freedom to create publicly accessible nodes with their own rules about payouts and service fees. So that capability would have to be removed to satisfy your proposal. We'd be better off if we can convince people to mine with p2pool on the basis of how profitable and how easy it is to set up and use.