As a Civil Engineer, I use spreadsheets to produce calculations. Diagrams form an integral part of the calculations, and there are several simple shapes that I find myself having to draw over and over again, from the limited "building blocks' of the shapes built into Excel.
It would be cool if custom sets of shapes could be added (e.g. for us Civil Engineers, 'I' beams, bolts, support symbols, "north" symbol, etc.).
For Electrical Engineers, a downloadable set of Elecrical symbols, and so on.