Assuming we're talking about a long period of time, they can only do that with your permission, otherwise they're effectively selling you a used car. Any time a dealer wants to pre-reg a car, they do it in their own name, or more dodgy dealers do it in the names of their employees. The car is immediately second hand then. When you come to sell the car, you'll be selling a 11/12 car instead off a 12/12 car and that will affect the value, albeit by a small amount so bugger them and their silly ideas
Now if we're talking a week or so then that's perfectly normal. My car was registered on Friday (last week) because that's when they got the car. I'll get the car a week later