It doesn't matter as much for marketing deals but it still matters. Marketing dollars would be most effective if they were spent targting the platform with the biggest sales. Marketing obviously has an effect or else it wouldn't be done. Don't you think someone would have noticed by now if say having an exclusive Destiny marketing deal didn't affect software sales?
You are thinking about this from the perspective of someone who closely follows games. I'm pretty sure advertising has little effect on you, and you already have a list of the games you want to buy. Most people aren't like that which is why advertising works. If they see a game they might like, but it is advertised for a system they don't have, or don't plan to get, they will discount that ad. They could do so because they simply don't want to get excited about a game they aren't sure they can play, or they could be console fanboys fighting the good fight.
Either way targeting an ad towards a specific console has costs associated with it. The smaller that console's userbase, the higher the associated costs.
As for why Sony and Microsoft have their strategies, my theory is that Sony is throwing some of their resources at Morpheus and combined with some game delays left a whole in their exclusive schedule. That forced them to emphasize third party more. Microsoft on the other hand needs exclusives more than Sony as an additional excuse to buy the XB1. It seems that they only emphasized their exclusive strategy, along with reduced cost, relatively recently once it became evident that the XB1 was falling way behind the PS4. And yes, I think that reduced marketshare increased their costs to do third party deals to the point where they weren't worth it to them.
E/ROTR budle over a Fallout 4 bundle if not simply a Halo 5/MCC basic bundle.