There are really no "cons" to this idea. If you wish to use it as a single back surround it would probably do quite well. For years before the days of PLllX, I used a single back surround speaker and even in a particularly long room like mine, the single speaker did its job. I have since gone to the double speaker arrangement, however, since those back speakers are primarily "fill", if you ask me if I can hear a noticeable difference between having one or two speakers in a back surround configuration, I can honestly say, not really. I would suspect that perhaps in a smaller room where the back wall is much closer to the listener, then one might hear a difference.

It is interesting to note that even the latest model AVR's have the capability of hooking up a single back surround in the manner you describe so it is obvious many, because of other issues, or maybe just space limitations and wiring want the flexibility of doing just that.