I like when I have two different signals. When you have the chance during playback to have something unique going on in the left ear that may not be happening in the right. Isn't that how it happens in real life? Aren't we trying to recreate a real life experience? You don't want to cheat the listerner, they are the most important. You want everyone to have the experience that mirrors that epic moment. If not epic, at the least, THE LEAST, they should feel as if they haven't ever been to that place but experienced exactly what it MAY be like.
How you record a sound is important and placement as well. The mic, bit depth, and filters. Think about if someone was making a sound to an ambulance that is driving by. Would you ever know from which direction the ambulance was coming from if there was no difference from ear to ear? If the sound didn't go from faint at one point to strong at another? That is equally as important for anything that is passing by or in the distance and is becoming closer.
As far as splitting into mono, grouping one sound with another is a different case. You are now trying to pick the best single signal that compliments the signal you are combining it with. Sometimes you lose what you loved about the stereo version of one signal, in order to create a new and unique stereo version of a new signal. All in all having a great recording session is important and knowing what best way to use the signals you have is equally important.