From Floyd's (and others) research, lower frequency driver placement in rooms is important, this also doesn't seem to generally be the same spot that you want to put your Left and Right speakers.
Also, bigger drivers designed for the lower frequencies usually do so with less distortion (If we can hear it or not is a whole other topic and thread on here).
Downsides are integration, more locations for speakers means different phase relationships between them, and it takes a lot to get them right, but when you do everything blends together to where you can't even tell there is a subwoofer.
I am a fan of using separate subwoofers and DSP to get them aligned properly.