Hedge, overkiling a creature isn't a problem. You're overlooking a lot of things, such as enchantments that can save a creature from a killing blow (Healing Charm and Bull Endurance), the overall usefulness of Piercing (useless against any Resilient, Incorporeal, or generic 0-armour creatures), and most importantly that the important thing isn't how much average damage is done but rather the probability of killing a target in a given number of attacks.
In your example, with the 8-HP/2-Armour target, the 4-die/2-pierce attack is less likely to kill to kill the target in two attacks than the 6-die/0-pierce attack. In a standard distribution, ~49.9% of the data will fall below the average. Roughly speaking, this means that the 4-die/2-pierce attack has a ~24.9% chance of rolling less than 4 damage on 4 attacks (where the mean is), but the 6-die/0-pierce attack has less than a ~24.9% chance of rolling less than 4 damage on both attacks (since the mean is above 4, one could roll below average on both attacks [say exactly 4 damage on both] and still kill the creature).
There are also other cases to consider, such as one attack rolling poorly but the other rolling above average. However, at any value of HP and 2 Armour the 6-die attack will have a greater probability of killing the creature than the 4-die/2-pierce attack does for pretty much the same reason as above. Plus the 6-die attack is more useful against things without armour as well.
Piercing does have value, but it isn't nearly as valuable as more dice. But the situations where going for more pierce rather than more dice are fairly uncommon and require that, for the same price, you would get more piercing than extra dice.
