OK. what makes your ideal tubeset (631?) "better" for this application?

more fun:
It's not a track frame.
- that's why i wrote "track". In my size at least it's the "trackiest" of all the other fat-tired road frames. higher BB, shorter chainstays/wheelbase, steeper STA/HTA than any of the other options (Jamie Roy, Paddy Wagon, 925, San Jose, One-Way, Fillmore...) all while being able to fit fatter tires than most of them.

Fenders? You can't mount fenders? Oh crap. then I must have done something wrong since i've got full fenders mounted on mine. P-clips work fine. Sure they're kinda ugly, but fenders are ugly to begin with. Yeah it would have saved me an hour and two trips to the hardware store if there were fender eyelets, but then again, unused eyelets are kinda ugly in themselves.

Millions of old road frames that'll accept fat tires? Show me. Where can I find an old 531 frame, with criterium geometry (not an old touring frame) that'll accept 35c's?

-Also, horizontal drops aren't needed for cross, you can always run a tensioner.
not if you want to ride it fixed. I'd argue that the Cross check is the most versatile frame currently on the market. Not much comes close.

-I thought the Karate Monkey always had a hanger
It does. A derailleur hanger on track ends. Now there are much more elegant solutions out there.