They should be allowed, I just don't think they should take it.
Don't see why not if there is better regulation, after all someone with a more expensive car would be paying more insurance in the first place... maybe some kind of incentive if you downscale possibly?
Because every time someone takes an expensive courtesy car, every single person pays for it with their premiums. If folks only ever took the cheapest thing, like an Aygo, then premiums would come down.