The LSAT is a perennial favorite for criticism. It doesn't have anything to do with law school or legal practice, and it measures nothing more than your ability to study for the LSAT. Neither of these criticisms is particularly good, but they still get made from year to year.
The LSAT tests two basic skills, logical reasoning and reading comprehension. Described like that, it's pretty easy to see what exactly the LSAT has to do with law school and legal practice. Not that there aren't plenty of lawyers lacking in either or both skills, but those lawyers are generally considered quite poor at their jobs.
As for the LSAT just testing whether you prepared adequately for the LSAT, law school and legal practice both require regularly require you to prepare for tests. If you can't spend a month or two learning how to master logic games, you probably lack the requisite skills to study for a law school exam or prepare for a trial.
The best defense of using the LSAT (in conjunction with UGPA) is that there really isn't anything better. It's not a perfect test, a lot of what it takes to be successful in law school and legal practice isn't tested. But, it is somewhat correlated with law school success, and it's all that we've got.
Somehow, despite all the known shortcomings of the LSAT, no one has bothered to look across campus at our medical counterparts and reach the obvious conclusion. Law school needs an equivalent test to the MCAT. A test of both basic intellectual skills, as the LSAT currently tests, and also basic knowledge required to get through law school, something equivalent to the bio-chem knowledge you need for the MCAT.
The design of the test is simple, it's a simplified version of the Multistate Bar Exam. None of the tricky questions or hard cases, just a test of really basic information. What are the elements of a contract? Difference between Murder I and Murder II? What are the five freedoms guaranteed by the First Amendment? Okay, that last one wouldn't be on the MBE, but it's still background knowledge you'd really want a 0L to have.
Content of the MBE, difficulty level of the MPRE.
Grading for the exam would have one significant difference from grading for the LSAT. The bottom performers, people getting less than half the questions right, don't get a score at all. Their grade is simply "Fail." They might get a numerical grade that they can see, to tell if they were even close to making the cutoff. When applying to law schools though, the only message the schools see is "This applicant is incapable of learning basic legal concepts."
If you want a test that will predict your ability to succeed in law school and in the workplace, this is it.
The obvious snag though is that we're saying to get in to law school you must already be good at law school. But, isn't that what law school is supposed to teach you to do? Aren't we putting the cart before the horse?
That's where the difficulty level comes in. It really would need to be as easy to pass as the MPRE. It has a wider variety of content, so you'll need to study for more than two hours the night before, but the questions would all be quite basic. You might struggle in memorizing all the rules, but there should be little or no difficulty in understanding the concepts.
To eliminate the need for expensive prep classes (students have enough financial burdens as it is), the LSAC or whoever administers this test would put a comprehensive guide online, available for free. Print it out, read it a few times, make flash cards, whatever. At the end of the day, if given all the resources you need you can't teach yourself the elements of a burglary, you really have no business being in law school.
The test would reduce the law student headcount, but it's not a artificial barrier to entry design to improve the careers of other students. It's a very meaningful test of competence. It's eliminating from law school students who are likely to fail out or if they do graduate, students who will not pass the bar exam. Those aren't people who would be competing in the job market anyways. Those are however people that law schools ought not to be able to take $120,000 in tuition from, and people who need a clear, official message that law school will lead to financial ruin.
As an added bonus, the baby MBE would make law school a bit more useful. With everyone entering having the same background knowledge of basic legal concepts, law schools can spend more time focusing on the meatier issues, the things you couldn't learn on your own as easily. Just imagine how incredibly useless medical school would be if the first year was spent catching students up on freshman level biology and chemistry. And imagine how much less frustrating your 1L classes would be if the professor could answer stupid questions with "this was on the entrance exam."