I like that you bring up the bad/weak manager topic and also the topic of knowing when to leave. Though bad jobs can be good for the career, they can be extremely detrimental to the esteem as well as the career if you don't figure out the right time to leave.

With a bad/weak manager you could be belittled on a daily basis in private or public. You could also be expected to accomplish things that have never been communicated or communicated entirely differently (I know that can happen despite the manager) and then be subject to the belittlement.

Granted, even with bad managers there is plenty to learn. For some, a bad manager means you learn how to manage and do the job for the bad manager. This can also teach one how to deal with adversity and work well under pressure. However, it is essential to keep your self aware of the situation and prepared to leave at the appropriate moment. Bad employers deserve as much loyalty from the employee as they give.