It depends on the organization.  Some organizations have light-weight project manages that don't really "lead" technically.

If the PM isn't supposed to be lightweight, then yes, they're not offering much value to the team.

If the PM is supposed to be lightweight, then they can offload some of the politics from the team.  It's the dev lead's (scrum master, technical lead, etc.) job to make sure this happens...

I wish I could count how many times I had one of those managers, unfortunately I don't have enough fingers.

@Nick: Perhaps one finger is all that's needed.

@Lee

Totally

@Peter: It depends on the organization.

I'm going to differ and here's why. Updating Gantt charts is in no way a useful task. In fact, as far as processes go, a Gantt chart should really be able to update itself.  

Assigning tasks is the same way.  The Gantt chart is supposed to encompass the tasks that need to be done and the people responsible for completing the tasks.

Think of it this way. Once the Gantt chart has been built, the project now has all of the required tasks and a complete timeline. I know what my tasks are. At the end of the day, I update the status or progress on these tasks and the Gantt chart should just update itself.  Stakeholders interested in the status of the project can look at the chart themselves or get a better formatted status report.

I'm not saying this is what actually happens, even if it should. What we're saying is that someone who spends *most* of their (expensive) time doing stuff for which a computer is better suited is not "adding value to the team".

Lightweight or not, you want project managers doing things like customer relations, reporting, quality control, training and change management. Updating task lists is a horrible way to spend time.