Why should one boil water over medium-high heat for pasta?
A few lines of thought:


Pasta water boiling over happens to the best of us. Either from not paying attention, using too little water, or using too much water. Using the medium-high heat versus max heat helps mitigate boil overs from happening.
On my own stove, when the burner is at its highest setting, much of the heat ends up being directed up the sides of the pot, rather than on the bottom of the bot. The gas flame becomes so wide underneath the pot, that much of the heat is lost to the air, rather than being focused on the pot. This makes the medium-high setting more efficient than high. Obviously this all depends on the pot and the burner, but even my larger pots have this happen to them.
If you were shooting for another temperature, like 200F (93C), instead of boiling it would be hard to hit that number on high heat without going over. Even if you have a thermometer in the pot and turned the burner off immediately after hitting 200F, the water would continue to rise in temperature as the residual heat carries over. Ideally, as the water gets hotter and closer to 200F, you would want to slowly back off the heat until getting to the point that will allow you to hold the water at exactly 200F. I think it's the same idea for boiling water. It's okay to use high heat, but you want to back off the burner as the water gets closer until you end up on medium-high: a good happy-medium for keeping water boiling versus having hotspots on the bottom of the pot.