Above all: I you need it, it's useful!
I don't know what's exactly on your mind with the task manager. It can be helpful to have the sources and tasks in one application. Normally you have an issue tracker application like Redmine, JIRA or Bugzilla to define and handle bugs/tasks and you can connect those applications with SCM-Manager.
Bugzilla on the other hand does only know bugs and not tasks, so it could be useful to have a task manager in SCM-Manager for Bugzilla users (and of course others).
Cheers,
Daniel