An interface (typically in Solidity, with full NatSpac) is necessary for articulating an ERC.
The reference implementation is a quality signal that shows the author actually took the time to read the thing they themselves wrote. I support such a quality signal and I think it is or should be a requirement for publishing an ERC. Otherwise, people are publishing bad ideas that they themselves did not even think through and it becomes my responsibility to point out their flaws… lest EIPs be polluted with low quality content.
It will be nice to link to immutable resources, I can support that push. This can include publishing an application, incorporating the reusable reference implementation on-chain. Ideally this should be on Mainnet, spending real money to deploy. Spending real money is also a trust signal.