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.