Quick Answer
If a school recommends AAMC PREview and is high on your list, taking the exam is usually worth considering if you can register, test, and receive your score without disrupting higher-priority parts of your application. A recommended status is not the same as a required status, but it is also not meaningless.
The key is timing. PREview is offered in fixed testing windows, registration closes before each window, and scores are released approximately 30 days after the testing window. If you can test early enough for your score to be available when schools review your application, a recommended score may provide another data point about your professional judgment.
If timing is impossible, recommended participation gives you more flexibility than required participation. A school that requires PREview may not consider an application complete until it receives a score. A school that recommends PREview may accept the application with or without one.