I agree that Halo is a system seller (really the only thing Sony has in the same sort of league is Gran Turismo), but in the hands of 343, I'm not so sure - if Halo 4 is anything to go by.
Who's to say people won't just get a PS4 and get Bungie's Destiny, especially if it plays more like Halo of old than 343s Halo games?
Plus, 1st party exclusives do have some differences to 3rd party ones - for one, they tend to utilise the hardware better, nor can they go multiplatform if their title is successful, like Bioshock and Mass Effect did.
And anyway, my original point was not that Sony exclusives can sell more consoles than MS exclusives, but that Sony's bevy of technically accomplished, first party devs are more appealing to core gamers, who care about titles like Journey, LittleBigPlanet etc, The Last Guardian etc.