Yes down force is nice but as in cars the general public is not into controlling things like a transmission or depth control. Many garden tractor implements work well with float. I have thought about putting the mechanical float in a different place on this rod so as to be able to lock it out when that is preferred like the hydraulic 54 snow plow.

 

I’m going to guess/hypothesize that even in the “useful” applications it would be a detriment, would be easy to reduce traction on the rear wheels and cause undue wear on implements even when trying to avoid doing so.

 

Instead of adding down force, just add weight to the implement. Gravity is free, and the hitch can lift quite a lot. The weight can be as simple as a CMU, I've been known to give my center blade some foot "english" to help it dig a bit.