They were used in vietnam. They were used during tet in artillery, fired horizontally
In wwii, IIRC, much of the bomb load was proximity charged, and HE.
In my thoughts, a bomb full of flechettes that projected all that shrapnel outward would be worse than useless against soldiers that were bunkered, foxholed, entrenched, or otherwise barricaded from horizontal fire. In the case of entrenched troops, a 400 pound HE bomb would be much more effective by blowing up the fortifications and releasing shrapnel than it would be to fill the bomber up with a payload that is more steel than it is explosives. But, in a war that even included the panjandrum, it would be wrong to deny something bases solely on whether or not it would be practical.