If you actually care to do the math, the accuracy gets increased only for the fang moves, so the expected values for the important moves (earthquake) only goes up by 3/2, and the moves that do get boosted have their expected values multiplied by 3/2*20/19, which is 30/19. With swords dance, the power of every attack doubles. Also, what's the point in using poison tail if it can learn poison jab?