I think it's a slippery slope for the developers. If one console gets exclusive DLC that the other one will never get you risk alienating a whole bunch of potential buyers. Not talking about small things like horse armor but extra levels. If it's a timed exclusive that's not a big deal because the other console will get it eventually. Look at the backlash Bungie was getting recently from some in the Xbox community who feel they helped Bungie be the powerhouse they are and felt it is a slap in the face for the PS4 to be getting all the extras for the upcoming Destiny. It is a business and they are having money thrown at them by Sony and MS and Sony obviously won this round but to the people buying the games they want to be shown that they are appreciated and, in this case, many felt they were not.
For me, I agree with Zack that as long as it isn't DLC that is needed to finish a game it really isn't a big deal. Or if it isn't something that helps flesh out the game story or an important mission that makes the story feel more whole, not a big deal. I think for games with the potential to play 100s of hours people will feel like they are having a great experience with or without the extras. For games that have a 6-8 hour sp experience and then the console you don't own is getting exclusive DLC that adds another couple of hours, well that could be a big deal.
Again, for myself, I've had games that I've loved but months later when DLC has been announced for it, I've already moved on and am no longer interested in revisiting that game again just for the extra couple of hours added. Have also bought a lot of DLC that I felt was a wasted of time and money.