A wider rear end will make the quad more stable assuming you have your shocks set up correctly, as the admin stated the wider a-arms will have more leverage over the rear shocks (again assuming you haven't adjusted them) and making them softer. This will also affect how well your stock rear sway bar works or doesn't.
So, you end up trading high speed stability (a wider quad) for something that corners better. Don't confuse SRA equipped quads with IRS either the two set up at the same width would require dramatically different riding styles.
Also, if you made the front end wider with wheel spacers and you didn't modify any of the stock steering you would quickly tire as you would suddenly have that much less leverage and when you hit something it would feel as though the bars were getting ripped out of your hands.