Personally I think while they aren't all appealing to the same demographics the same amount they are obviously in the same market. Someone that buys a wii is less likely to also buy a 360/ps3. Fanboys on all sides like to argue to make their system sell the best. And whatever console is last has the most rabid fanboys as they need to try hardest.
E.g with the gc it outsold the PS2 and xbox one november and nintendo fanboys went crazy saying this was going to be a trend bla bla bla. PS3 fanboys constantly bring up the arguement of attachment rate (this is possibly the most retarded) to try and make it seems games did better on it than 360. Can't think of a 360 one at the moment, maybe any week it outperforms the PS3 they act like it's going to continue forever.
P.S. The reason I say the attachment rate arguement is so stupid is because of course a fking new system with a small install base has a high attach rate! Especially an extremely expensive system. Who buys it first? Hardcore gamers. Who buys the most games? Hardcore gamers. Trying to argue that X game did this much on the PS3 with only X install base. So clearly if the install base was twice as big the game would perform twice as well is just painfully ignorant of any sort of logic.