I think it is at best a questionable practice. Even assuming applicants from the same school crowd out each other (which is not a given; in son’s class of less than 50 there were two students admitted EA to MIT and Caltech each), having put the work (and the money) into the applications, one deserves to have all their options on the table, because, conceivably, they may change their mind.
We were thinking son was going to MIT for sure until he got GT’s Stamps semifinalist invite. Now we’re not as sure. Who knows what else may change between December and May. He is keeping his Princeton, Stanford, and a few other applications in the pot. (Although he did toss Yale and Columbia applications that weren’t yet submitted)