I do buy Nintendo consoles because of Nintendo games but it does not mean I will only buy their games.
My current Wii-U library is :
Nintendo : Nintendo Land, Pikimin 3, SM3DW, DKC:TF, Wonderful 101
3rd Party : Lego City Undercover, Zombi-U, AC:III, Darksiders II, COD:Ghost
However I agree with the OP, the "mass" Nintendo buyers are the family/party gamers that will only care about Nintendo games. That is why 3rd party games sell so poorly exceptiosn being party games (Dance Dance, etc...) or family games (lego, ...).