For example, fuel left out in the sun is often contaminated with water.
No it does not, only an open cap would do that.
It only says that UV exposure causes nitromethane to change to di-nitromethane, and that it is "more interesting" to use. Unfortunately, it doesn't go into detail about WHY it is more interesting. Hmmmm....
di-nitromethane is contact sensitive, but the methanol will help prevent it from breaking down. The major problem with leaving out a jug in the sun is that the pressure will break the seal on the cap. This will later show up as a collapsed jug, and sometimes some air will leak in. Not a problem as long as the cap is tightened. Also the some dye can turn dark, this is not a problem as the fuel will still run well.