If you combine that rule with the ability to endlessly stack Draw 2 or Draw 4 cards, you have what I think may be the most evil version of Uno that can be played. We called it Thermo-Nuclear Uno (i.e. Mutually Assured Destruction) since you knew that dropping a Draw 4 would almost certainly lead to someone being buried in cards and it might even be you. Even if you manage to dodge the stack, payback is coming and you won't get lucky every time. I can remember games that we would finally just quit after an hour since no one would ever be able lay down their last card. Merging two decks can make this even more brutal and cuts down on interplay shuffles too.