I can't think of a legitimate reason to be allowed on DC's
except DCs aren't all out of town bypasses. They can be upgraded arterial roads, smack bang in the middle of the city and generally a reasonable way of getting where you want to go without heading miles out of your way or having to use ratruns (quite often presenting their own dangers)
I acknowledge your points tinas, I personally don't want to go and ride every 70mph DC in the country just cos I can, 50s are scary enough for me and pretty boring to ride TBH (ignoring near death experiences) but if you start banning cyclists from roads without a damn good reason you're marginalising cyclists even more than they are now.
Well designed DCs should be reasonably safe places to be, good sight lines, few junctions, multi lane so fairly easy to overtake properly. All we need is for people to drive appropriatley, these aren't Mways, there are slow moving vehicles about watch what you are doing and drive safe. Not rocket science.