Posters are correct: The yield rate as well as the admit rate will eventually come out. It is required (I think by law, but I am not sure) in the Common Data Set.
I agree with bicoastalusa that schools play games–not only with the yield rate but also the admit rate. Some schools will not admit a candidate because the candidate seems too strong and is likely to go elsewhere. Why admit a kid you will almost surely lose: it will hurt both your admit rate and yield rate. And those are two things admissions officials are apparently judged on.
The problem is that if schools stopped publishing yield rates, as bicoastalusa wants, you still have the incentive effects of the admit rate. If you don’t publish yield rates (which I don’t think anyone is proposing), then admissions officials are largely unmonitored. Prospective applicants would also be denied valuable information about their prospects. I agree there is a problem, but I’m not sure that suppressing information will help or hurt.
My proposal would be to make all information, and I mean all information, public. Each and every application file with the candidate’s name, recommendations (with names), etc. That way prospective students could get a much better feel if they are a viable candidate. Schools would have far fewer incentives to play games. Of course, such a proposal will never be implemented in the U.S. (Something close to this policy can be found in some of the Nordic countries, however.) Among other reasons, do you think politicians or very large donors want the public to know how much of a break their kids receive in the admissions process?