I do a lot of charting, and defined ranges are completely nifty for use in specifying the Series formulas. If I didn't use them, I would have to spend a lot of time each month updating the Series formulas.
I do a lot of linking between individual cells in the 15 workbooks in my financial system. It sure helps to not have to care if I move some of these cells around for various reasons. The other workbooks don't know about the moves because they link to the defined names.