Hi, I can understand why the Team field is locked to stop someone deleting it, but it restricts how it can be used.
Firstly, I can't setup any value, unless I go to Portfolio and set them up first. Common practise when you have multiple teams is to have a Teams common field which you then assign Stories to. I can't do this with the Portfolio field.
Secondly in an enterprise there will be a lot of Teams, and I don't really want the entire list available to all project. I usually setup a field context applied to projects which then has options on which are the available teams who can work on those projects. That way there is a restricted list available in each project and its very difficult to choose an incorrect team.