Heat control and boiling

Have you ever heard it said about someone with no culinary skills that they "can't boil water"? Hyperbole aside, boiling water isn't all that great. In fact, there are only two things that you are ever supposed to boil - pasta and shell fish - and you only boil shell fish in order to kill them as quickly and humanely as possible.
The reason why you don't boil more things is because it is violent. Whatever is floating in the liquid is tossed around, flung into the sides of the pot and into the other things in the boiling liquid. Hard "boiled" eggs crack, vegetables overcook, potatoes begin to disintegrate. Don't believe me? Try it. Take a potato, cut it up into even pieces, and put half into a pot that you bring to (and keep at) a boil; take the other half and put it into a pot that you bring to a boil and then reduce to a simmer. Sure the boiling potatoes will be done faster, but when you strain them, you'll have bits and pieces gumming up the colander. The simmered potatoes will retain their shape (as well as better flavor and mouth feel due to the starch's reaction to the lower cooking temp, but more on that another time).
The lesson here is that faster isn't always better, especially when it comes to food.