The rewards are one. Granted, they're not huge (I also use Bing and find it amounts to about 1 $5 Amazon gift card per month), but if you find the result quality indistinguishable, it's enough. Also, like sremani, I want to drive innovation in the area of search, and while lots of people pay lip service to wanting more competition, putting your money where your mouth is by not using Google is the only real way to do that.
True, I really do like the rewards. But I really think Bing would get rid of them if it would was a good financial decision. And I think it would be a decent decision if Bing was doing better.
I'm sure that Microsoft has a great deal of data telling them that the Rewards program has a positive ROI, especially because a lot of the rewards are sweepstakes which essentially cost them nothing. Also, I have watched other people use Bing and seen four-digit numbers in their rewards count on the top-right. (It's agonizing, like watching someone who doesn't know about tab-completion use the command line.) This tells me that the number of people who use the Rewards is quite small and this contributes to its usefulness.
