I would love to be able to use % progress in a "formula" custom field. Here is my use case:
I have progress tracking enabled for all of our projects. We invoice our projects based on percent complete. For example, if we have a $100 project and we are 43% complete, we would invoice for $43. If I could make a "formula" custom field that would take our project price and multiply it against our progress, it would basically generate an invoice for us. I think there are other use cases too!