Big Bad Bob said:
No, that's exactly the point. He used a significant augmentation to hit (+20 bonus) against a foe that it was largely wasted on (low AC, not too many hp) and then refused to use a similar boost to the damage.
I strongly object to house rules intended to patch bad decisions.
Either it was important enough to kill the goblin that the player should not have held back on the damage; or it was not that important, and not worth the use of the True Strike spell in the first place.
There are lots of ways to get an attack bonus that is much higher than the target AC of creatures of your level. That's what Power Attack is for -- shifting excess attack ability into damage. Of course, you risk missing of you take too much off the attack bonus. For archers, instead you get Rapid Shot and Manyshot -- the opportunity to take a self-imposed hit penalty in order to do more damage. Unlike PA, you don't get to tune the adjustments, but you also usually get more extra damage than a simple 1-for-1 or 2-for-1.
If getting high enough attack rolls would guarantee death or extra damage, why bother doing anything to increase damage ? Smart people would focus entirely on boosting their attack bonuses. Suddenly, the cost of magic items granting True Strike over and over would become less of a deterrant, because there is now an added benefit due to a house rule.
If you still want a variant that would make big hits more deadly, instead of adding more damage, take a look in Unearthed Arcana and see if some of the tweaks to the Massive Damage rules appeal to you. If the 3 hp was more than half the Goblin's total, you could call for a Fort save to avoid instant death. Of course, any such rule should apply to the PCs as well.
