When reducing, why do you simmer instead of boil? All recipes call for simmering a dish if you wish to reduce. I know that if for example if I wish to reduce wine, it will take me double the time or even more if I simmer instead of just cranking the heat up to a boil. But there must be a reason for this - what is that? There is only a 6c difference between a simmer and a boil.

I also presume boiling is more permissive in some cases than others, so an ability to distinguish the importance of simmering is necessary to know when you can crank the heat up to save time.
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).