There is always some resistance to the idea of a standing hammerfist, its from our cultural association with boxing i think. I find the hammer very effective both standing and on the ground. Standing takes some getting used to, but its very good for the hands if you hit the forehead and keeps the hands forward ready to defend the takedown. mark hatmaker teaches the use of standing hammers in his MMA instructionals. Not many MMA people beleive in them because MMA is basically made up of the existing combat sports (boxing, kickboxing, wrestling/judo, BJJ) and those styles of striking do not allow hammers in the sport. Though i think the older puglism and muay thai arts had them.
I have been woprking on an interpretation of Pankration for a while now and we spar with the hammers, i also use them in my personal MMA training. Thye just add another dimension to your striking.
Here is a movie of our recent Pankration bouts at ARMA-SFL, there is some nice hammers at the end of the video
Pankration Bouts 42 mb