Lead nurturing is essentially about handing leads over to sales at the right point - when the prospect is ready to buy.
Running a marketing campaign and pushing anyone you hear breathing over to sales is fraught with problems. A successful sales person knows they should focus on qualified opportunities, so anything that's not at the ready to buy stage will inevitably be dropped. That's not to say they would never be ready to buy, just that they aren't at this point in time. Nonetheless, the result is that they may well never buy from your company.