[johncowan]

Not if it's so constraining to the implementer that you can't get any implementations. Nobody's paying us to do this stuff.

[bjoli]

Of course not, but knowing that the program should never continue executing in an erroneous state is for me a pretty nice-to-have. Scheme enforces correctness and safety in many places and I think that one in particular is important enough to warrant being opt-out rather than opt-in.