You're probably right TJ. But I just figured that if your application was good enough to get a deferral, as opposed to an outright rejection, then your application is at least competitive with the accepted RD applicants. Based on that, I thought that an acceptance rate of 20% would be too low. On second thought, it's probably wrong to automatically assume that all of the deferrals would be as qualified as the RD applicants.
However, I don't really see any advantage in getting a deferral. Just speaking from my own experience and what I've read on CC, I think there's too much false hope put on the opportunity to improve your application. I don't think I would have applied EA if I didn't get that feedback from my rep.