Er, if you can relate to the box idea for freqs spacing, then using stereo can add an extra dimension so that you have depth. This means that you can place a sound, which lets say is in the freq range 5000 - 6000, and another sound around the same feqs but at a different width. You can do this to many elements, but i always leave my bass and kick in mono.

I like the psp plugins personally, but it deppends on what effect im after.