I get around this by creating a mono dummy bus in Audio Connections Outputs (dummy because it isn’t actually routed to any physical output). When I render a mono file I first assign the output of the track to this dummy bus. The resulting rendered file is in mono.
You could render in place and then use Menu “Audio”—> Convert Channels… and set to “Singel Channel” in the dialogue. that´s the quickest way that I can think of.
There is no disadvantage using the stereo file, unless you’re really tight on hard drive space. Personally I wouldn’t worry about it and just use the rendered stereo file. As was pointed out, the audio signal within that stereo file is mono.