When reducing, why do you simmer instead of boil?
There's another reason for not boiling liquids, besides the possibility of making a mess (boiling over) or ruining it (scorching, etc.).
You actually reduce the amount of flavor by boiling.  As Kenji explains on Serious Eats :

But here's the deal: when simmering, water is not the only thing escaping. Ever notice how when you come home to a pot of sauce simmering on the stovetop or perhaps a beautiful pot roast braising in the oven, your entire home smells of it?
Guess what: if those flavorful aromatic compounds are reaching your nose, it means they are leaving the pot.

So ... if the only goal was to make there be less liquid, boiling's fine.  But if you actually want to concentrate the flavors, you want a slow simmer.
The article also goes into more details about reducing alcohol, and some problems with boiling tomato sauce (which I believe applies to most pectin &amp; other hydrocolloid thickened sauces).