Why adjusting story points in carried-over stories is bad practice:
1) the backlog is a historical record of the work done, and the estimates that were made. Adjusting the story points down makes it look on the record as if the team had underestimated the story
2) Velocity is a representation of how much work the team get through on average. By adjusting the story points you are effectively making work disappear. Velocity is an approximate figure, one should never look at velocity within a single sprint as meaningful, it’s the average velocity over a number of sprints that gives an indicator of how much work to bring in.
3) Story points are a tool for the team to facilitate estimation, nothing more.
By taking points out of a story you are reducing the apparent average velocity of the team and making the reporting incorrect over time. It may make the burndown chart look better in an individual sprint, but the long term effects on reporting are harmful. The sprint burndown chart is no business of anyone outside the team anyway.
