Another advantage to training hammer fists is if you have something clutched in your hand (ie. keys, flashlight, roll of quarters, etc) you can instinctively strike while maintaining hold of said item. Of which, those items can/will become a weapon and the motions for using those items as a weapon are essentially the same as the hammer fist.
Also, there is something to be said for the satisfaction of smashing something really hard with a hammer fist vs an edge of hand; while they might appear the same, the differences are there.
