It's true that if the dispatcher expects exactly one argument, you can't add optional arguments. The solution is very simple: don't make dispatchers which expect exactly n arguments. Make all your dispatchers take rest parameters which they ignore. In fact, 'multi-keyed already does exactly that, so if you used (multi-keyed area 'shape), your example would work exactly as expected. It's not a hard fix.
Regarding reasons for not implementing Clojure-style multimethods in Arc-F, how about this one: because you can implement them in plain old arc! Don't put stuff in Arc-F if it can be done in Arc or Anarki already. Interoperability is good - we don't need to fork the community as well as the language.
It's not just the number of arguments. One advantage of CLOS-style multimethods is this:
(def bam (a b)
(err "collision type unknown!"))
(defm bam ((t a ship) b)
(defm bam ((t a ship) (t b missile))
Because of the computation of keys, you can't exactly implement the above using clojure-style multimethods without tricks like method bouncing.
Like I said: already implemented clojure-style multimethods. And tried it. So yes: I'll continue bashing my head implementing CLOS-style multimethods, because while clojure-style multimethods are cute, they're not good enough for all cases. Arguably neither are CLOS-style multimethods, but at least we have an alternative choice.
Also, there's a good reason for implementing this in the scheme-side: efficiency. Your implementation of clojure-multimethods allocates a cons cell for each argument to the multimethod. The scheme-side implementation does not, because on the scheme-side I have access to the ar-funcall* functions.
Efficiency of course is not a concern, except when it is.
As an aside, there are several bits of Arc-F that look like they're implemented in Arc, but are actually implemented in the scheme-side. There's an Arc implementation of them in arc.arc, which is labeled as "IDEAL", while the actual binding to the scheme side is labeled as "REAL". For example, the basic compose function is actually implemented in the scheme-side, but there's a reference Arc implementation in arc.arc marked "IDEAL". The only reason they're on the scheme-side is due to efficiency. Ideally, they would be in Arc: realistically, they are better implemented in the base system.
This is true, and indeed I mentioned it in the OP; you can't do type-based dispatch with clojure-style multimethods except on exact matches. You also can't get method chaining. But clojure-style has the advantage of being damn simple, and easily permits dispatch based on non-type-based conditions. CLOS style has the advantage of being more flexible about dispatch and integrating well with OO methodologies.
I'm not trying to convince you that CLOS multimethods are bad or not to implement them; a full implementation for Arc or Arc-F would be _very cool_. CLOS is without a doubt my favorite thing about Common Lisp. But Clojure-style multimethods are not "cute" or useless. They're just not a universal panacea. Very little is.
For me, cute means something really really nice, not necessarily useless. Like cute mature women, for example. Or better: cute girls, with guns. LOL.
Method chaining may require us to rethink PG's type system, at least if we want to handle a drop-down to a more generic type (which arguably is the more interesting bit about chaining). It's reasonably easy to drop from the "argument 2 matched type T" to "argument 2 matched no types", but how about when we want to drop from "argument 2 matched derived type D" to "argument 2 matched base type B"?
We would have to have an operator which determines if D is derived from some random type B, and forcibly standardizing on it. This is going to make my head explode.