When I make a symbol out of an object, I should be able to lock certain size properties. For an example, if I have a standard button saved as a symbol, instances of that button should have always the same height but with variable widths. In that case, I would lock the height attribute of that button so on resize only the width is changed.

This can help with complex symbols too where some dimensions cannot be changed or the symbol will be distorted so the designer could lock all of them to ensure that future use does not distort that symbol.