I can't reply for Mark, but the commercial lisps have a development experience that is hard to match. When using CLOS you get an extra smalltalky experience whi

I can't reply for Mark, but the commercial lisps have a development experience that is hard to match. When using CLOS you get an extra smalltalky experience which the other CL implementations lack. And the debuggers are superior in just about every way. Racket is nice and all that, but the commercial CLs are just that: commercial, aimed at people writing large programs with very high demands. Anecdote: I once hit a weird bug when trying to get a very old CL program running. Nothing big, but once in a blue moon I would get the wrong result. I spent 3 days banging my head angainst it, trying to find a way to trigger it reliably. I inspected the live environment, did print-debigging. A friend encountered the same bug when running the program in Allegro CL and managed to debug it within minutes.