As you aware admins can give bonus points directly from user-profile page in frontend. However, each given bonus points are not recorded, instead it is overriden. This should be improved. It is not the proper way.
Suppose that admin decides to give +50 points to a user via bonus points field. After a while admins again gives +75 points who a reason to the same user. In such a case admin should override previous given score and should consider it, so admins should give 75+50 points. After a time this working approach becomes very complicated. The admin cannot give when the first time bonus was given.
In this respect, I believe this should be improved by core hack or at least by plugin.