I don't think the problem is simply "gamers want to justify an investment" - if this were true I'd expect a more rational level of discourse. Instead, you see emotionally driven often blatantly fallacious (from the classic ad hom to the ever common straw man) arguments, designed to aggrandize one side and tear down another.
At the end of the day, costs of ownership aside, any of the consoles can generate a fun experience. When comparing the 360 to the PS3 you are, in all actuality, handed what amounts a nearly identical game set. Sure there are a handful of exclusives on each side, but those games are hardly worth donning the armor of idiocy and embarking on a grand crusade to save the souls of the poor, depraved heretics on the other side.
I think perhaps the better explanation of the phenomenon can be found in the inherent drive of people to belong to a group and feel superior to those around them. PC gamers perch atop their technical marvels and croon that their platform is superior because of advanced technology, leading them to the assumption that they are superior intellectually (because it takes nothing short of genius to assemble and operate such a masterwork of silicon) and have a more refined sense of taste (because they are willing to expend exorbitant sums of money to feed their habit) in games. XBox owners praise their often dysfunctional device for it's vast library of quality games and handful of blockbuster exclusives while decrying PS3 owners because of the smaller library, supposedly inferior exclusives and robbery level price tag. PS3 owners exalt their obsidian monoliths because it is technologically superior and boasts more utility than the system being used by the XBox loving peasantry, all while believing that THEIR exclusives are inherently superior to those "enjoyed" by others.
This is the same kind of discourse you see among the fans of rival sports teams. My team is better because we have X Y and Z stars which are clearly superior to players A, B and C who are simply giant wastes of money. My team has more heart and/or skill, your team is nothing but wannabees and crybabys. People are decried as fair weather fans for liking the team that is successful and it's seen as a badge of honor to continue to pay money to watch/attend games/buy merchandise for a team that hasn't been successful in living memory. There is no logic to be found here, but people seem to increase their enjoyment by joining a faction without rhyme or reason.