I want to be clear that I think CLAs can be a bad idea for contributors and people should think carefully before signing them. But since the AGPL doesn't obligate you to sign a CLA, I don't see how the existence of CLAs is an indictment of the AGPL or of dual-licensed business models. It's my belief that most dual-licensed software wouldn't be open sourced at all (and a lot of it wouldn't exist at all) without the dual-license model that Kuhn opposes.
CLAs are frequently coercive, rights depriving and generally a bad move, and I as a FOSS contributor won't sign them.
Yes, occasionally they are used correctly - but more often than not they're used to crowdsource work on what is really a proprietary codebase with crazy potential restrictions if the owning company decides to call them in.