- You consider that the user getting their account pwnd is their problem. Let them set their password. That includes things like forums and other websites where an user losing their account is not critical and you can easily just tell them "too bad, use a better password next time" if something goes wrong.
- You consider that the user getting their account pwnd is going to be your problem. You're a bank, or some paid service for instance, and the user will ask for support if they lose access to their account. In this case it 100% makes sense to force a password on the user to prevent them from selecting something stupid IMO.