What exactly is the "good argument for a single source" other than its cheaper?

I would, and do, use relatively cheap subwoofers. When I say that I have five, I am including the three woofers in my front three channels. I do not use small speakers hear and the front three speakers have very good low end capability - down to about 35 Hz. There are two subwoofers wired into the LFE channel, one at the front near the three mains and one at the rear. So at LF, with the Dolby selection for the front set to "large" I do have five woofers. The front right and left are closed box full range, the center is port, and the subs should be bandpass. Also keep in mind that this kind of low end power can easily over power the response if the room is not well damped at LF. It is almost impossible to do an optimum loudspeaker design that is independent of the room. So when discussing "right and wrong" we should always consider the room when talking about the speakers.