I never understood why ArcMap clears selections when you stop editing. You may do a bunch of selections, realize you're editing the wrong data, change editing and then have to redo all of your selections again. Or, what bothers me most often, have a selection of some features while editing, save your map, close ArcMap and it asks if you want to save your map. Even though you just did save. The reason it asks is because the first thing it did when you closed ArcMap was to stop the current edit session. This clears your selection which is a change in your map so it asks to save again, this time without your selections. At the very least there should be an option to not clear selections when editing is stopped.