YouDontHasToCallMeJohnson
So, why is MONO important? NOT!

1. There are many iPod docks sold that are MONO only (1 speaker)... and these sell regularly at costco, walmart, and many other locations. I will lump in the "iPod Dock Alarm Clock" with this as well.
2. FM Radio collapses to MONO when the signal weakens. Some people live in this "collapse" area...
3. You will be amazed at how many people listen to music through the internal speaker on an iPhone or equivalent.
4. Many (if not most) 70 volt systems are setup in mono. These are the systems in department stores, elevators, commercial buildings, etc... etc...
5. The majority of PA systems are still setup in MONO. This is done to assure that the people on the right side of the stage hear the same thing as the people in the left, or middle.
6. Many TV's are still MONO, and AM Radio is MONO.
7. If your competitor prepared for MONO and you didn't... your music will sound terrible compared to theirs when heard in that environment.

 
If you are 100% certain your music will never be played in the above environments, then go ahead - ignore mono.
 
My rule of thumb is this...

1. Never assume your music is being listened to in stereo.
2. Mix it in Stereo, FIX it in Mono! The stereo mix WILL sound better when you fix any problems that get revealed in mono.
3. If you care about your music, you will make it so it will still sound the best it can in the worst possible situation.

IMO - the only reason not to make your mix work in MONO is laziness!

I love my mono little friend, one day I may by it a brother, when I chain it, it will still be mono, if I by a splitter it will be stereo.

All natural sounds are mono (apart from that produced by the Stereo Feedback Echoless Duck).

Stereo is just an artificial virtualisation of the 3D space containing mono soundsources.

So both are important and related I reckon.

Did I forgot my meds again..?

I feel that mono is bigger now than any time in history since the 70s. Phones, docking stations, portable Bluetooth speakers, Tivoli-type desktop radios etc etc...

Ronan gives a convincing argument for mono compatibility here:http://recordingbootcamp.com/2015/05/the-best-stereo-technique-for-recording-strings-on-pop-rock-productions/
 
Rocky

mono ROX

ston
All natural sounds are mono (apart from that produced by the Stereo Feedback Echoless Duck).

Stereo is just an artificial virtualisation of the 3D space containing mono soundsources.

So both are important and related I reckon.

Did I forgot my meds again..?

How do you mean 'all natural sounds', I guess you don't mean instruments? For instance an acoustic guitar is resonating and giving off different frequencies from teh stings and the body. so has a stereo width. Same to a flute, the head is producing the sound, but the sound is escaping from holes along it's body and the body is also resonating differently along its length. A piano too has strings spread throughout it's body and even when playing one string sympathetic resonance occur throughout the whole instrument.

Ronan gives a convincing argument for mono compatibility here:http://recordingbootcamp.com/2015/05/the-best-stereo-technique-for-recording-strings-on-pop-rock-productions/ 

 
That is some good advice.  Especially in the context of a pop record.  If you need to mess with the stereo field, recording with a coincident pair is a good idea.
 
One of the reasons we do opt for spaced pairs in orchestral recordings is the stereo separation based on phase is much more natural sounding and open, and less exaggerated than a separation based on level differences, but we do have to sacrifice good mono compatibility in AB or Decca arrangements.  One of the reasons US engineers in the mid 20th century adopted the Blumlein pattern for ensemble recording over Decca and AB which was popular in Britain and Europe, was because of Mono compatibility.  The US and US consumers have always been a little behind the times when it came to audio technology, and we still are to a certain degree.  
 

Kamikaze
How do you mean 'all natural sounds', I guess you don't mean instruments? For instance an acoustic guitar is resonating and giving off different frequencies from teh stings and the body. so has a stereo width. Same to a flute, the head is producing the sound, but the sound is escaping from holes along it's body and the body is also resonating differently along its length. A piano too has strings spread throughout it's body and even when playing one string sympathetic resonance occur throughout the whole instrument.

Stereo reproduction is a means to emulate the effect of hearing sound sources in 3D space (nominally 2D, but with some additional processing trickery 3D emulation is possible).  All mono sound sources produce a stereo effect, or have stereo width, due to the fact that we have two ears.  Stereo reproduction systems emulate this.
 
As to whether a guitar, flute, piano or 'produce sounds in stereo' as you state is a debatable point.  In this case, it's our ears which produce the stereo effect based on the transmission of sound waves from, effectively, an infinite number of individual (mono) sound sources.  In this case, by infinite I mean continuous, or as near continuous as you can get given the limitations of certainty in measurement.
 
The sum of those individual sound sources do not have stereo width, they have 4-dimensional width.  That we have two ears provides the means to identify the location in 3D space, a simplified version of which is location in 2D (stereo) space.
 
To put that another way, a guitar|piano|flute doesn't produce a stereo image, it produces an incredibly complex set of information allowing, given sufficiently accurate measurement and calculation, depiction of the entire 4D environment.  For example, you could build an image of the room the instrument was in, the materials the room was constructed of, the location  of the instrument in the room, a depiction of how each element of the instrument was moving in space etc.  Lord knows how much processing that would take, or the accuracy of the information gathering that was required.  You'd need something like...a human brain (or a Culture Mind) ;-)
 
So I'll correct my original statement from 'all natural sound sources are mono' to 'all natural sound sources are 4-dimensional'.  Stereo they are not.
 

OK.
So MONO and Stereo are here to stay for a while.
 
Mixing for either should lend and ear to the other.
 
And depending upon the particular project, 5.1, 7.1 and 10.2, should be "collapsible" to all the lessers
 
What prompted my post was the often asked question about using headphones for mixing. It seems to be a requirement now.
 
Recently I was listening to TV with headphones and was very happy to hear how guitar sound was ping-ponging from left to right, quickly, in time.
 
And:  I learned a lot listening to the Beatles White album, especially Dear Prudence, with headphones.
 
---------------
Definitions are fun.
 
If a tree falls in the woods and no one (human) is within ear shot, does it make a sound?
Yes.
 
Does it make a noise?
No.