Re: Would the Ikkyo pin work in MMA?
To piggy-back on what Chris and Kevin were saying, it is critically important to understand that the men who developed traditional Japanese martial arts were concerned with weapons, multiple attackers, and surprise. There tactics and techniques were designed to answer these problems.
Face down pins do offer less control of the body, and in my experience they are more difficult to achieve, but they allow you to control one arm and position the opponent's body between you and his other arm. If he has another weapon, or a hidden weapon you didn't initially see, he has to come across his body to hurt you. This gives you time to do what is necessary to further subdue him.
Face up pins and rear mount don't offer that protection. Additionally, if you are armed, these pins will place you in greater danger of having your weapons grabbed by your opponent.
There are some techniques that make great sense in a one-on-one, empty-handed, controlled environment that address the realities of a more open environment.
On the other hand, there are some techniques that aren't necessary for one-on-one, empty-hand, and don't properly take advantages of the "short cuts" that are available in that environment.
It's hard to put the square peg in the round hole.