Defendants settle to buy finality. A deal that leaves thousands of claims alive buys nothing, so global settlements routinely include walkaway (or 'blow') provisions: below the participation threshold, the defendant can walk. In the only systematic study of publicly available mass-tort settlement agreements, every one contained a walkaway provision requiring 85–100% participation.
MDL settlements are private contracts — no judge reviews them for fairness the way class settlements are reviewed. Your individual decision, aggregated with everyone else's, is the only ratification mechanism there is.
Sources & attribution