Actually what he said isn't completely incorrect in a practical sense...If you are replacing the front speakers and putting in a sub and amp, the difference between replacing the rear speakers and not replacing the rear speakers won't be audible.
I bought a pair of pioneers and replaced the front two door speakers, and then had an amp and sub installed. Sounds amazing. Once you offload the bass from the speakers the quality goes through the roof.
I respectfully disagree.
If you get high quality speakers up front and back....there is a major difference.
If you have a cheap, over boomy sub, then perhaps you may be drowning them out, or they aren't properly calibrated. The rears offer great immersion.