Seems about right, most middle of the road knives seem to fall under that category. SAK's are about 56, Frosts are 57-59, and Ka-Bar's are 57 on the Rockwell scale.
I think it you harden a knife too far past that point, field sharpening becomes more of a chore than a quick touch up task.
Because, the trick is to get a blade to walk that fine line between edge retention and ease of sharpening, isn't it?
Adam