Interesting replies.
Of course I was referring to sponsors motivating us to increase our promotion of them, rather than a general motivation to work. In fact most things seem to be negative factors. You see an email box on the tour, and you move on. The sponsor has a $75 promo day, and one thinks "I'll skip that then, and I'll have a look next week".
I agree that sponsors do too much for affiliates, and I suspect they grab a chunk of our traffic in payment. I'd much prefer an honest and simple arrangement where we do more work, and we get credit for all the sales we find.