I know where you guys are coming from as I like quinellas too, but only when I can't split two neddies for a straight out bet. The problem with estimating odds for novelties is not the mathematics. It's the fact that you have different pools and the TAB skim is higher for novelties. Not sure of the exact numbers but it's something like 17 to 18% for Win & Place, 20% or more for Quins and Tris upto 25% plus for Superfectas etc. Maybe someone here has more exact knowledge on this. The point being, if you are struggling to stay afloat on the punt, those few % points can sink you.
(Scuse the pun.)
Cheers