The goal is that the composable parts get woven into an efficient, planner-friendly query. Stored procedures completely undermine that unless something very exciting has happened since last I checked (SQL Server, but probably applies to all of them). You will likely end up in a "row by agonizing row" situation.
(Okay, maybe you can find a few cases where stored procs work decently, but in general both composability and performance will be much worse than the proposed functors.)
(Okay, maybe you can find a few cases where stored procs work decently, but in general both composability and performance will be much worse than the proposed functors.)