most people dont use rear speakers at all... they are quite pointless in a vast majority of installs.
for starters, you can easily put a set of components up front, in the floors, dash, doors, a-pillars, etc... while in the rear you are pretty much limited to coaxials or point sources. you can put components back there, but the aiming is harder and they are usually in a less than ideal area.
also, if you have a proper front stage, the imaging should carry to the rear seats with no problem. in my car my front component set is just as loud in the rear seats as it is in the front.
thirdly, rear speakers will pull the sound stage towards the rear of a vehicle. while this is only important to audiophiles and competition vehicles, it is still something to consider, and it goes along the same principle as the point made above this one.
lastly, they do nothing but consume extra power and money. why spend money on something that you dont really need? and why spend even more money sending power to it?
you are better off(in most installs, remember) only spending the money on a quality set of front speakers, and putting nothing in the rear. and one last final thought to leave you with... how often do you drive your car from the back seat? //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif