Since I've already talked about this a few times I'm just going to copy and paste. However, this is just my reasoning and opinion. I don't claim it to be the end all. It's one viewpoint on diags. I'm sure others have their's for different reasons.
+As many have mentioned diags are completely worthless. They are in no way representative of the real test. The Kaplan one I took was much tougher, scaled differently, different number of questions, different format, missing the writing section, different timing, and it wasn't on the computer. Diags don't even tell you your weaknesses since they are so different from the real thing.+ All diags tell you is that you aren't ready to take the test yet. Well is that honestly a surprise? It's not like you have been studying for the real thing months before you took this. It tells you what you already know, you have to actually study for the MCAT. Shock! What a surprise! You can't take the MCAT without studying first. Diags are meant to demean you to get you to study more and so that they can keep their promise of raising your score x amount of points. Take a practice test after you've done some studying, then you'll be able to accurately access your progress.
Now let's say this isn't Kaplan's diag or another diag, but one of their actual practice tests. In that case, it should be structured the same so cut out the stuff in between the + signs. Even a practice test doesn't accurately tell you your weaknesses, though it may be in a similar format to the MCAT. To describe my reason for this I'll give an example. Let's say you're really good at physics, but you haven't taken a course in awhile. At the same time you're bad at bio, but you just finished a course. On the practice test you're bio would probably be higher because it's fresher. That said, once you start studying, your physics skills will have the rust taken off of them and you'll start seeing huge gains. Unfortunately, since your practice test said your physics was bad, you've been focusing on that instead of bio, your real weakness. Sure this isn't a good example, but hopefully it gets my point across.
I think you should hold off on any practice tests until after a month of studying. Then during the second month take two-three tests. Finally, in the third month, take as many as possible. While I think you should hold off on practice tests, you SHOULD do as many practice problems/passages as humanly possible. Practicing problems is very important (imo the most important part of studying for the MCAT). If TPR has timed practice section tests, take those as well. Save the practice tests for later so you can get your rhythm and accurately access your weaknesses after you've covered everyone. To me, the key is to save most of the practice tests for the last stretch.
. How bad is this? 
