Nowadays, it's very unlikely that your essay will be looked upon favorably if you use it for every school. Adcoms aren't stupid; they know if applicants are truly interested in their school. Plus, this is Yale... I think it's worth the extra effort.
<p>It really depends on what essay you're talking about. The basic CommonApp essay doesn't have anything to do with the specific school, it's just about showing schools who you are. So it's fine to use that for all your schools. You probably shouldn't recycle the "Why I want to go to XYZ University" essays, because you want to be very specific, but it's not the worst thing in the world. I used the same essays for Penn and Northwestern, with only the names of the cities, the programs I wanted to be in and the extracurricular clubs I liked, changed. I got into both. Yale was my top choice, though, so my essay for them (really a short-answer) was very specific.</p>