It sounds better done that way, in my opinion. And if you try it and don't think so, you can still copy it and flatten the copy to mono, so it's not like you're losing takes by trying:
Almost all vocals are recorded (on tape) in mono, with the same signal going to both L-R channels. A common effect is to then run a bit of stereo reverb on the recording, but this means that technicaly it is no longer mono. That's the reason that sometimes when you use a vocal remover on a song, you get a "ghost" vocal that you can't get rid of.
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Another reason is that plain mono tends to sound rather "flat" compared to stereo. If you have backing instruments in stereo and you add your own mono vocals and it don't sound right, then try adding reverb to your vocal track (stereo reverb if you can) or record the vocal again in stereo, with the mics a couple feet away and 3-4 inches apart. Stick a pop filter way out where they're supposed to hold their head, so the singer doesn't end up crowding the mics. I point cardioids at the center of the pop filter instead of crossing them 90
or 120 degrees, or use A-B omni's. You don't get a lot of separation but you will get some, and it does make a difference you can hear. If you don't like how it sounds, just flatten the L-R channels to mono.
-One of the biggest problems trying this is singers who throw their head around, tho'.
