I think I must re-iterate the balance issue. No, not play balance...
One of the main factors in creating a blade, a longsword or even a rapier or saber for fencing, is that you have to have the balance point just right. If it's off by an inch, that blade won't function well in combat. Talk to someone who fences a lot and they'll tell you a good balance point is as important as the blade's weight.
With a mercurial weapon, the balance point is continually changing. Yes, a mace may not have an excellent balance point when it comes to parrying blows, but one of the critical things is that in a mace, the balance point is CONSTANT! A changing balance point on a sword goes against one of the basic fundamentals of sword making. Now, if you imposed a -4 AC penalty to anyone using it, then maybe I could see the mechanic reflecting reality, but as is, errated or no...
BAD!