Masteries Explorer vs Tough Skin

Hey guys, in the most guides and in the statistics on i see that tough skin gets picked over explorer in the most times. But im actually wondering why. 2 Dmg sounds like nothing for me on the other hand when you are chasing someone through the river or through a bush +15 movement speed can make the difference if you kill someone and get in range or if he gets away and i think +15 is really a lot for just 1 masterie point. Can someone explain my why tough skin is better than explorer ?
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