Have they ever explained why a contributor agreement is crucial? Will it assign copyright to RubyCentral?
My understanding is that having a legally enforceable contract help dissuade malevolent actions (easier to sue for breach of contract? But IANAL).
As for copyright attribution, I doubt it. That ship has sailed, that code base already contains contributions from hundreds of people, and I can’t really imagine a business model that would rely on relicensing.