Read that thread above it has got some very info in it. Especially form me!
Yes even two speakers in mono is not quite the same as a single mono speaker. You are relying on the mono phantom image in the centre which may not be strong enough. A single point source small speaker physically in the middle is actually better.
I actually permanently sum L+R being fed from a spare headphone out using a Hi Fi amp from a long time ago which is fitted with a mono switch on it.
Use you mix cube down at lower volumes too and you will hear any parts of your mix that are not well placed level wise just jump out at you. (or disappear)
Mono is also great for checking big wide sounds and effects to see how well they collapse down into mono. It is possible to get a good mono collapse and a wide stereo version on your mains at the same time. Also with mixing when you get everything really well balanced in the mono speaker down soft, up loud on your mains in stereo it will sound stellar usually.