In planning budgets, I need to roll up costs into something like this:
I can put the formula sum(children()) into the rollup field, but I have to do it manually and I have to do it for all of the parent and grandparent fields. I can't make it a column formula because then I can't enter the values for the children.
It would be nice if there was a column type called "rollup" or something that accepted numbers in "leaf children" fields and automatically produced rollups for "parent" (and "grandparent" etc) fields.
There is precedent for this - it's how the "% Complete" fields work. This is exactly the same - simpler even, since it just need to sum the children.