Rj,
I understand what you're asking. As far as I know, the performance difference of a pistol caliber cartridge out of a rifle is noticeable, but not super great. I'm not sure in the case of the CX4, but I'd expect it might gain another 100-200fps muzzle velocity. I could be wrong, though. This does warrant a little research.
For home defense, some nice +p defensive loads would benefit from a little more velocity, as this would help them expand more reliably. The added sight radius would also help, as would the form factor - it's easier to be accurate with a rifle than with a pistol. Also mounting some sort of holographic or dot sight would also help at night/low light, as would mounting a flashlight.
I think the advantages are several, however ammunition performance gain is probably at best not very great over a pistol. Think of it this way, though - it's more important to put your rounds accuratly into your target, than it is to have some whizbang super powerful exploding tip hollowpoint teflon armor peircing thermonuclear rounds! (basically, being able to hit your target more easily is enough of a performance gain!)