Peter,
I'm reminded of a management maxim that I use, "you get what you measure." For example if you have very few high severity cases, but they are weighted heavily everyone will want to work on those issues. This becomes even truer if my rewards are based on the same weighting. So if sev 1s are weighted higher sev 3s then the 3s will become less interesting to the staff. When generating a report to management you should be aware that you are setting the standard by which the team knows they will be measured and think through the impact of this report on the individuals you are reporting about. They will know how they are being measured and will respond to that measurement.
In my experience most severity guidelines are based on the impact to the operation not the difficulty of the task. Severity should set the priority of the work but my goal should be getting the most work done (done being completed and accurate and high quality) in the least amount of time.
Make sure your report includes information about both goals: Priority and Work Accomplished.
