It's not a matter of seeing a previous version. There are a multitude of AAMC practice tests that are quite similar to the real thing, so it's not like you walk into the testing center not knowing what you're getting into.
It's a matter of not being prepared enough the first time to do as well as you would like. Yeah sometimes people just have bad days, but the majority of people who prepare adequately will perform within +/- 2 points of their practice test average. I know plenty of pre-meds who didn't do enough practice tests or just did the free AAMC one (by far the easiest and usually overestimates your score) and then they walk out of the test wondering what went wrong. Not enough preparation.
If you took the current MCAT and retake it as the new MCAT, you still have to do better on the new MCAT to be any benefit.