Do you think that a project manager has to work as a business development/consultant for the projects too?

I think Yes, it is one of the main success factor for any project.

Yes, for the benefit of the project.

No I do not think it is good for a project manager to also act as a consultant for a project. The Project manager's role is to initaite and co-ordinate all activities of the Project from the stage of conception to completion.

He might not have the expertise required of a consultant needed for the project.