Plastic & metal expand and contract at different rates. The body filler in the middle would be pulled one way and the other, then crack.
Molding 2 unlike materials together is just asking for problems in autobody. Not to mention that step where the flare meets the quarter would have a valley of filler or panel bond atleast 3/8" thick, in and of itself just asking for a crack. You shouldnt be paving that much filler on a car.