That's an example of coin flips being reasonable, and good. Obviously, the flip is necessary to make Gengar's ability fair, and if fails, it's not like you're losing anything more than you would.
A good question is...how do you use Fainting Spell 6 times a game and still win? ^-^; Since Gengar does need to get knocked out to use the ability.
Still doesn't excuse a few of them, however. Well, more like half of them. I'm sure the incident that happened to inspire this topic was over a card that wasn't like Machamp or Gengar.
Was still a good example of the positive side of coin flips though. It's amazing what you can do when you not only think of what you can rely on, but what small factors can change the game around, without possibility of ruining your chances.
So uber rare, so uber-broken.