I think the bigger issue with "clean throws" in competition is that the opponent knows them, too. There's a reason MMA doesn't see most of the throws found in traditional arts. Many of those throws are hard to pull off on someone who knows them well, especially if they have a chance to watch you (or video of you) and see what throws you like. A single-leg is less efficient (from the standpoint of energy usage), but the mechanics in it aren't as easily countered, even by an experienced practitioner.This is the problem for "sport". If you slap on the mat, it's very clear that you have admitted that your opponent have just thrown you. If you don't and use that slapping arm to drag your opponent down to the ground, it may sound like both of you lose balance together.
This is why it's very difficult to see a "clean throw" in tournament. No matter how perfect your throw may be, your opponent will try to drag you down.