i think that people use them for 1 of 2 reasons:
1. Being that they have high dps
2. The specials you can use with them
they are only .25 speed slower than a regular mace
regular mace = 12-14 dmg
diamond mace= 14-17 dmg
put it this way: you use this mace have the concusion blow special primed...explo flame strike and say the wep hits em too....most likely they are either one shot or very very close to it.