Its because the blunt and edged attacks are very annoyingly tied to AI behavior.
If there is any EDGE attack present, it will use that attack as a primary attack, but if all the attacks are BLUNT it will only use the actual main attack a 4th of the time, making the weapon rather useless unless your using it as an adventurer.
You may have noticed all the 'edged' blunt attacks have little to no penetration depth, which will make them behave like a blunt weapon, only the combat announcements get weird here and there. Its not a perfect solution but I figured it was the least imperfect one hehe.
@Taffer: awesome, thanks! I will have a look once I get home. The modest mod joints shouldn't have a huge impact on combat as they cannot actually be target with regular attacks, though it would be good to check if they don't break too easily when covered in armor. I will have a look at that sometime soon and put out as an optional patch.
In case anyone is curious, I have been mucking around with replacing specific weapon skills with general skills for one-handed, two-handed and polearm weapons (shamelessly ripping off mount & blade? maaaybe...), as I always found it somewhat stupid that my dwarves have to completely re-learn fighting when switching from an axe to a hammer.
It does have some undesirable consequences, such as wood cutting needing some workarounds, and I have been considering merging the two-handed and polearms skills just to get the NPC equipment to make some sense for once. (right now NPCs with two-handed weapons always lug around a shield as well and use the weapon one-handed)
If I have something I find satisfactory I may put up a poll to see if anyone actually wants this.