I upvoted you 'cause Semi-Arc's doing what you want it to do (which is what really matters), but I'd like to mention that I actually kinda appreciate the way Arc accepts mismatched destructuring lengths.
I've seen other people's code use a (let (a b c) nil ...) idiom as a shortcut for (with (a nil b nil c nil) ...). As for me, I like being able to design multiple types so that they can be used in an ad-hoc polymorphic way like this:
Both these techniques could still be used in an Arc implementation with a stricter treatment of destructuring: The first could be done by defining a 'w/nil macro, and the second could be expressed as (let (x y . rest) rep.point ...). In your favor, the strict treatment makes it easier to write functions that do complain if the length is incorrect, encouraging fail-fast code. However, as far as portability is concerned, the "N args expected" errors are a quirk of Semi-Arc.
I'm going to try out Semi-Arc right now. ^_^ I'm sure I'll have some error reports for you in a moment. Hopefully not too many! :-p