A graph that shows when piercing is better than +2 dice for many combinations of dice and armor would be very helpful to this discussion. Tables of numbers are great but it can be hard to see patterns in them.
Not sure if this totally satisfies what you were looking for but from the
damage calculator I have made a table of the situations when it will be preferable to cast Piercing Strike (+3 piercing) over Power Strike (+2 melee) on one of your creatures. Below is a table of the differences in the expected damage dealt by a creature with the power strike incantation versus a creature with the piercing strike incantation (positive values mean that power strike is expected to deliver more damage than piercing strike):
| Base Attack | No Armour | 1 Armour | 2 Armour | 3 Armour | 4 Armour | 5 Armour |
| 1 | 2 | 1.296 | 0.815 | 0.611 | 0.856 | 1.005 |
| 2 | 2 | 1.198 | 0.593 | 0.259 | 0.642 | 0.914 |
| 3 | 2 | 1.132 | 0.428 | -0.029 | 0.404 | 0.76 |
| 4 | 2 | 1.088 | 0.307 | -0.259 | 0.168 | 0.569 |
| 5 | 2 | 1.059 | 0.219 | -0.44 | -0.046 | 0.364 |
| 6 | 2 | 1.039 | 0.156 | -0.581 | -0.234 | 0.162 |
| 7 | 2 | 1.026 | 0.111 | -0.688 | -0.394 | -0.028 |
| 8 | 2 | 1.017 | 0.078 | -0.769 | -0.525 | -0.199 |
| 9 | 2 | 1.012 | 0.055 | -0.83 | -0.632 | -0.349 |
| 10 | 2 | 1.008 | 0.039 | -0.876 | -0.717 | -0.476 |
As you can see from the table, in general, power strike is much more useful than piercing strike. However, if your creature has a base attack of at least 3 dice and you are coming up against a heavily armoured opponent then piercing strike has some situational uses.
A couple of caveats when interpreting the table: firstly I've assumed that the attacker does not already have some form of piercing ability, if the attacker does have this piercing ability then deduct one from the armour for each point of piercing when looking up the defender column on the table to decide if piercing strike will be a better play than power strike. Also, although it is relatively easy to do so on the damage calculator, I haven't yet created a table for the addition of Veteran's belt. I suspect that the addition of Veteran's Belt will swing things much more in favour of piercing strike.
