After using inventor for years in machine design, I've found the frequent need to copy values out of the measure window and into an external application for quick calculations. There are a few minor annoyances in doing this, such as the measured unit copying along with the value, which Windows calculator does not accept.
Having a built-in calculator within the measure window would be very useful for making these quick calculations. I can't imagine it would be a painstaking addition to the user interface. There does exist an 'add to accumulated value' tool, which is great but other operational buttons (+, -, x, etc.) are frequently needed in my case. This could be shown in the Measure window with a small 'formula' or 'results' sub-window that can be broken off the measure window to store these results between measurements.
My method thus far has been to copy values from Inventor into a separate notes window only to remove the unit and recopy the plain value into Windows calculator. If this feature was built directly into Inventor, I would expect it'd save myself and many others time.