The variety of experience leads onto learning more from fights. If you fought the same thing again and again you don't learn anything new and you stagnate in your fighting. That is why Zak never managed to think linearly enough to find a counter to the double parry low. Fighting against different foes also keeps you on edge instead of expecting the same thing every time, and as before with learning new things. Fighting a drow isn't the only thing that matters, how can it? How does only fighting one thing make some one better? That's like saying the best Karateka is the best fighter of all martial arts because he's beat all other Karateka. Leading on from that Drizzt isn't just any drow, he doesn't fight like every other drow.