Actually, fuel won't usually evaporate much of the good stuff or absorb much water overnight. And it also won't do much of that when almost sealed in the tank. Overnight isn't much problem. But leave it more than overnight and you could have problems after awhile. It's not a good idea to get into the habit of leaving fuel overnight just because "overnight" very often winds up being "next weekend" or "next month".

Some nitro mixes need igniters in the fuel. Those are often very volatile. They wick off and the nitro won't pop. So leaving fuel in the tank certainly has no positive value.

BTW, a lot of times, the fuel you see left in your tank is darker simply because muffler pressure lines feed residue into it. The stuff is darker from obvious reasons and is thicker because a lot of the light components have burned away in the combustion chamber.

If you run a brass clunk and brass tubing in the tank, leaving that in raw fuel isn't a great idea. Alcohol and nitro do a number on the brass. And it does more damage when the brass is immersed in the fuel.

It's worthwhile to pump out your tank for another reason. You almost always clear the lines when you're doing it. And fuel that's had a chance to evaporate in the lines does more to screw up the next engine restart than the stuff left in the tank. Of course, overnight won't do much of this but...............