One example where this is a problem. I buy items for a customer. I open the Vendor Invoice for the item to use to fill out the Customer Invoice. However, once I open the Vendor Invoice, it's MODAL and I can't open anything else. I should be able to have BOTH side-by-side on my screen. Not everyone runs everything full screen where they can only see one item at a time.
This applies to ALL sub windows that don't require that they be answered, such as error messages. So, this applies to opening Reports also.