If this generation mimicks the last one, then the most installed console user base will be the standard for which all games are developed. Personally I think the 360s weaker hardware last generation led to a lot of stagnation in game development because of this fact. IF the xbox one falls behind the ps4 this gen, they could be relegated to receiving inferior versions of every major multiplat title for this entire generation, much like Sony was stuck with last gen.
That's why it matters I think, more than anything. These two companies are battling for which console will be considered the standard against which games are developed and if that standard becomes the ps4, with its slightly better hardware, then the xbox one could suffer for this entire generation as a result. That will hurt its sales for years.