Yes, the symmetrical cooling of the HD and BMW cylinders may have something to do with it. Even the Yamaha XS engine which Junkyard Dog races with aluminum sleeves and Nikasil coating has a cam chain tunnel between the barrels so should be oil cooled.
Jim Comstock tried them and I don't recall hearing about any distortion issues, only cost to benefit but I don't recall Jim stating what clearances he ran.
From John Ellis's experience it sounds like the big advantage is being able to close up piston skirt to cylinder wall clearance.
Just my humble opinion but distortion may or may not be an issue with aluminum sleeves in aluminum Norton barrels. Who is ready to experiment?