It would be useful to have a sum that only included values from list items the user has read access to.
So if you have a list with items A, B and C, and one user only has access to A and B, they would still be able to see a total of their view (A and B).
Why this would help: When using selective access, not all views roll up nicely, and it would be nice to have a totals item even if a user doesn't have read access to all the list items.