I know I would have used Moltensteel Dragon in my prerelease deck (if I had attended prerelease). But unless I was running red, I might not have thought to use Immolating Souleater. Who wants to use 2 life to do 1 damage to the other guy? Any fast, aggressive, potentially sucessful deck, of course.
You can use regular mountains in a pinch, or the 2 life, and you can do it as many times as you want. It's an activated ability and you can choose to use that ability before the attack phase, after the attack phase, or at any point during the attack phase.
If you have the opportunity to cause lethal damage to your opponent, then you have the opportunity at any point to immediately win. A smart guy named Russel Tassicker points out (along with about 1,001 other awesome observations) that this is just like the "Channel Fireball" combo shenanigans people used to pull around my high school cafeteria, only with just one card.
Of course, I'm sure the only time I ever try this my Hound will be met with a swift Dismember. But who is above taking a few risks in the name of victory?