The balance shafts are not meant to balance the rotating mass (crank, rods, pistons, etc.) of the engine. They are used to smooth out the power pulses inherent in a four cylinder engine. Here's why: From a monetary standpoint it would be cheaper to balance the engine without the shafts (less parts and machining). Even if it was meant to balance the rotating mass it would not get rid of the imbalance, it would just counteract it. Therefore it would still be out of balance but you wouldn't feel it in your seat. In other terms, the balance shafts cause a vibration in the block that cancels out the vibration caused by the rotating mass.
So, I would expect an increase in engine life mostly from an increase in oil supply to the crankshaft.
I'd like to get to the bottom of this one so if someone has good info on this, lets hear it. If you think I'm wrong, I'd like to know.
Thanks!