Is it good for the Project Manager to be a subject matter expert ?

Being a subject matter expert will certainly help a good project manager but it is not a prerequisite for his/her success.

sure it's good but not must as long as the PM does not forget his/her main role of managing the project and not being the SME.

The Project Manager must have sufficient knowledge of the subject under discussion and that is why we call him subject expert

Yes,being a project manager you must have the subject knowledge of your project !

The project manager should have the knowledge that make him aware of all project areas

Not really necessary

I think being SME can help you getting respect from the Project team. You can use it to manage the project more efficeintly. Also, you can identify potential issues more quickly as you are happen to be a SME.

However, if you have an excellent and motivated team then your team will help you resolving the issues and you can just focus on the Project Management activities. In this case being SME won't give you much of the benefit.