On the AAMC exams I do the writing section. I have a few reasons:
1. It builds your stamina. The real exam is five hours long, not four. Furthermore, the writing section is stressful since you have to have to come up with an argument and decently sized essay in 30 minutes, twice. If you're interested in simulating exam conditions at all, you have to do the writing section. You won't get the option to skip it during the real test (well, okay, you can if you don't mind getting a J), so it's best to get used to having to do it so during the real thing you aren't half-focusing on how much you don't want to be doing the WS.
2. Practing the WS will help you iron out any issues before the exam and get a good template going so you don't have to spend much time planning out an essay during the exam. The last thing you should want to is to have your first experience with the writing section be during your real MCAT.
I only ever practice WS during practice tests though. It's not nearly important enough to warrant taking time away from studying for BS, PS, or VR.