Exclusives are very important for the industry and for competition (especially on the hardware front). Exclusive console development has always been about utilising the strengths of a single platform for the benefit of your game and it's provided us with some of the best games of all time. When a game goes multiplatform you can't take full advantage of the hardware available whilst having to deal with the disadvantages of both platforms.
Of course the business decision to release on multiple platforms makes sense, but I think for gamers, exclusives tend to give more wow factor as they push the hardware or utilise in a way that just wouldn't be possible with multiplatform development.