It drags and drops onto something on the screen, something that you see in your viewport obviously, not some place random. Therefore if you make a mistake you will see it. And CTRL+Z takes care of mistakes well enough.blah blah blah
Plus it's not like level artists were excited monkeys, they show great precision and thoroughness in their work, so it's highly unlikely they're going to miss a wrong material applied in the area they are working in before they hit rebuild.