If you can make your various instruments have a space of their own in mono (I'm not good at this yet) then things can really open up in stereo. I have a mono button on my monitor controller so it's really easy to switch back on forth
also unless you are going to be listening in the sweet spot between your monitors, most music you hear is going to be more or less in mono so it's worth knowing if it'll hold up. for example internet radio or streaming on a cell phone speaker, usually mono, lap tops, ipod/mp3 docks etc ,the speakers are so close together that it's not possible to put your ears into the stereo sweet spot so once you are a couple of feet away it's no longer stereo, your basically hearing it summed to mono.
If you aim for commercial success it's arguably even more important to have a good mono sound. Music at the mall and other venues where it's pumped in = mono, a lot of TV and radio stations receive a true stereo signal but by the time it reaches you it's often faux stereo or mono and so on
We think we live in a stereo age but unless you hear everything through earphones or are always perfectly placed between two speakers, most of what we hear is more likely to be mono