Pierce is the skill which negates Defense. The attack stat is linear for pierce and increases the pierce by 0.145% for every attack point the base pierce is 1.6%; which at 300 attack gives 45.1% pierce. With the upcoming Episode 4 where the Stat Point cap will be increased to 340, the maximum pierce will be 50.9%.
Probability is the skill which negates Evasion . At 298 Attack the increased probability due to the stat is 4.51% wich is 0.01513 Probability Per Attack Point, but at 40 Attack, the bonus probability is 0.72% which is 0.018 Probability Per attack. And at 2 attack points it is 0.08 Probability Per Attack Point.
The attack stat does not appear increase probability linearly, but rather it appears to increase in a Negative Exponential fashion.
It is currently unknown if at 0 attack stat there is a base bonus probability or not. If there is a base bonus probability then the relationship between Attack and Probability is linear. As the base probability would cause a higher degree of error at the lower attack points, but with the higher attack points, the percentage error is far more reduced.
But if there is no base probability then that would mean the relationship between Attack and Probability is Negative Exponential.
The attack stat increases the minimum and maximum damage output of both the standard and advanced weapon classes. Because of the extra damage, this increases DPS (Damage Per Second) which increases the rate at which mobs are killed, so increases the rate of grind. The amount it changes the damage output by has yet to be worked out though.