gswitz
@aric
It will matter very little how many speakers you use.

It matters a lot. Firstly if you have two speakers setup and you switch your mix into mono then you are relying on the centre phantom mono image to appear and do the work. Most of you have your speakers too far apart (having seen many pictures of your setups) Or your speakers are not the right distance apart. Hence the phantom mono image will be weak and not that helpful.
 
Sending a L+R mix to a single speaker and preferably it being in the centre physically makes a huge difference.
 
Also two speakers in mono will still create some sense of stereo simply because the sound won't be arriving at your ears at exactly the same time. With one speaker this goes away.
 
Something like the Avantone speaker is ideal. 
 
http://www.avantonepro.com/Avantone-Active-MixCube-Powered-Full-Range-Mini-Reference-Monitors.html
 
A pair of these is fine for monitoring stereo mixes on a small speaker but still not great for mono. A single one however serves both applications perfectly.  It solves the mono problem and still lets you hear your mix on a small reference speaker.
 
Another reason I like to not use my main speakers for this is simply it is just great to instantly hear your mix on another speaker and if its small like this then you get all the advantages of hearing your mixes through a speaker bottleneck which is another story again but will reveal lots of problems with your mixes which can be easily adjusted and then when you do go back up onto your main monitors often you will find the adjustments you made in the small speaker won't be that obvious in the larger speakers only now you are satisfying two speaker systems instead of just one.  (although I must also say sometimes when you get things sounding really good in the small speaker your mix can sound stellar on the larger monitors too)
 
A single point mono source though is simply hard to beat. The people that say you don't need it have obviously never heard it and if they did and worked with it a lot then they would realise what I am saying about a single point source speaker is actually very true and great.

@Konrad...
idk if it's true, but I feel that the percentage of people listening in cars (where the speakers are spread very wide) and with headphones has gone way up since the 60s. In the 60s I feel most people were listening to car radios in mono and those listening in stereo were likely listening in a room with the speakers on the other side of the room not too far apart. Thus panning 100% was relative for most listeners.
 
When you have headphones on, 100% is 100%.
 
@Jeff
 
You always teach me a lot.
 
You make good sense.

Nitrox32
Hi Jeff,
Thanks for the response.  I've heard lots of good things about the Avantone but it looks like they are only sold in pairs.  Is there a good monitor that doesn't come in pairs?
 Aric

 
I am pretty sure you can buy just one.  And passive ones too for that matter if you happen to have a spare amp lying around which I do. Much cheaper. 

Amazon has single powered Avantones here for $249. It's $10 cheaper if a go in with a buddy and buy the pair.