Arguments for, or against, a topic chosen at random. Submitted as an essay, and graded by at least two independent readers. It may not scale, but it's a better metric.
So you learn how to game essay-writing competitions. Big whoop. The most brainwashed people I know are arts graduates, who think they know "how to think".
In reality, they know how to use hollow rhetoric to make a convincing case for their own prejudices, and present straw-man arguments to pretend you've covered both sides of an issue. They also find some way to "twist" the question, if it's easy.
The most important part of essay writing is not to be a perfectionist, and try to think too much. You come up with a few points (supported by examples, augments, or references - depending on the requirements), throw in some formulatic meta-discussion (i.e. '... though it may be more important to ask "why", not "if" ...') and get the fucker done. Don't worry to much if you're right or not, just make the bloody sausage. Oh, and try to predict what the markers (generally soft-left yuppies, who went from school to college back to school) will want to hear, or will get their attention - they have dozens (or hundreds) of similar papers to read and do not want to think.
My point isn't that essay writing is necessarily harmful. It's just another mostly useless technique to learn, and probably adds up to something if you can mix it with other techniques, and a little knowledge.
Tests are used because a metric is useful for identifying weak schools so that they may be improved. Do you have a better metric?