setting remote_user at top level of playbook has no effect on tasks!?

There is something I do not understand about remote_user.

When I set a remote_user at the top level of a playbook, when it comes to tasks, it seems to use my user name on the invoking machine (there is a similarly named user on the target) to execute the task unless I also specify the desired remote_user at the task level. Is that the expected behaviour? If so, what is the use of setting remote_user at the top level? Is it simply to log in?

Thanks for any clarifications. BTW I use the latest git cloned version of Ansible and my target is a Debian 7 server.

Can you show me an example playbook and output that illustrates this problem? (Possibly suggest running with -vvv to show the users being used in each step).

Thanks!