Listening to your mix in mono is a great practice that will help you point out potential pitfalls. How?
Here are a couple of things to listen to:
If you have extreme stereo effects going on, you might find some elements cancelled when the mix is summed to mono. Listen carefully, switching back and forth between mono and stereo – is anything getting lost? Does the balance of the mix change too much? If something is wrong, adjust in mono and then listen back in stereo.
The Low End
Does your bass get boomy when the mix plays in mono? If so you probably have some unnecessary low end content happening on the edges of the stereo image. You might not really notice it when listening in stereo but once that stereo low end adds up with the bass you already have going in the middle, the low end gets boomy and loses definition.
Again, adjust the low end in mono until it starts to sound good. Then switch back to stereo and notice how the mix magically sounds better.
Anyone making club music should make sure their mixdowns sound good in mono as a lot of PA systems are in fact mono.
In addition – if you can achieve a mixdown that sounds nice and balanced in mono, you can be sure it will sound even better in stereo.