One argument for standardized tests is that the classroom allows people with high social capital but low academic ability many ways to bullshit their way to higher grades.
Taking practice tests and familiarization with standardized tests helps, but a person who can't test their way out of a paper bag can do very little to improve their standardized test scores other than hire a ringer to take the test for them.
I think a mixture is probably the best, because, cynically, I bet people with high social capital could engineer a way to improve their standardized grades as well. By requiring them to cover more bases, it drives their costs up. A ringer is an obvious solution, but there is a lot of room to be "creative." The best way for admissions to pick well is to always remain liquid on metrics. Keep the pool wide; diversify your metrics portfolio.
Taking practice tests and familiarization with standardized tests helps, but a person who can't test their way out of a paper bag can do very little to improve their standardized test scores other than hire a ringer to take the test for them.