Hi Thomas,
The teams are the ones asking for this. The issue is that the velocity charts look at "what is" rather than what was true at the beginning of a sprint. They can plan their sprint and lock in the plan, but lose site of what that plan was if anything is added, removed, or re-estimated. Looking back over a few sprints, you can't easily identify how well you're meeting your sprint commitments if the contents of the sprint keep shifting.
I agree, these types of reports are for team improvement only, and only for each team itself, not for someone else to use as a stick against the team! It would just be nice to have a better way to look at historical information. It would also save us from having to split unfinished stories at the end of each sprint, which has its own complications and challenges...