here is another guy explaining the goodness of the two speaker set up

But the bigger reason is sound quality, largely an imaging issue..
If you don't care about imaging and staging, stop reading here!

However, if you do, you might want to start understanding it...

Imaging is a matter of psychoacoustics, fooling your brain into thinking there is a live performance in front of it, with a height, depth, and width..

If you want to create the impression of a stage in front of you, you don't have sound coming from behind you...
Which is why, at a minimum, people probably have advised you to run your rear speakers at sub-audible levels from the front seat...

Furthermore, there are phasing anomolies that really do a number on imaging, without really causing perceptible damage to the frequency response, although it does that too, but in smaller "bandwidth" chunks per interference point...

What I mean is that your front speakers will have one pathlength to your head.. the distance from speaker to your ears.
Your rear speakers will have a different pathlength.
And frequencies all have their own wavelengths, the lower the frequency, the longer the wavelength.

Frequencies combine when they are in phase, cancel when they are out of phase, and combine or cancel to varying degrees depending on how in phase or out of phase they are.

When you have two speakers of inherently different pathlengths playing the same frequencies (like front and rear speakers), at certain frequencies the wavelengths will line up and combine properly, at other frequencies they will combine negatively and cancel to some degree.

The higher the frequency, the more often in the frequency spectrum this happens... more phasing anomolies per octave. Which is why people may tell you to at a minimum, low-pass your rear speakers...

The biggest damage though is that your subconscious is not fooled, the "image" is not there like it could be.

Try this experiment at home, to see how phasing fools your subconscious, without necessarily seeming to mess up frequency response...

Sit between two speakers at home, equal distance. Play some good reference music, or even better, talk radio.
(And verify your speakers are wired properly first!!)
The voice (or music) should sound well centered, the voice eminating from exactly between the speakers.

Now, shut the stereo off, and flip the phase on one speaker (swap the + and - wires). Now listen again.
See?
The music and voice still sound the same, you can hear all the content, haven't completely seemed to lose any tones..
But now there is no "image".. it sounds diffused, unrealistic, smeared, right?

THAT'S the sort of damage phasing anomolies can cause...
And THAT'S the reason you don't want to run rear speakers if you don't have to!