There IS no erase & install option on ---> UPGRADE <--- DVDs.
You appear very sure of yourself, my friend - it makes NO difference whether or not Leopard was on my HDD before I upgrade, because it will be replaced by SL *after* the upgrade, which brings me to the very same situation I would be in, had I bought the FULL RETAIL DVD. I would not be able to use Leopard in EITHER situation, because I would have installed Snow Leopard, so what, prey tell, is the difference?. So you think I should go out and buy the Box Set, when I already own, & am licenced to use iLife '09?.
You're just being very silly indeed, and I suggest that you make good use of your high horse, before it falls lame.