Our use case is that we set a number of points for each task when we estimate them and we use the board view to organize our iteration's work. We can assign points with a custom field at the moment, but they're of little use if they're not surfaced in the board view.
The team knows how many points it can handle per iteration, thus having a sum for each column in the board makes the estimation/iteration planning task very straightforward.
The simplest way to handle this, imho, would be the ability to pick a custom field to sum in the board column's settings.