2-State Wall AC Switch
A mains switch that can stay in the middle state is a bad quality switch.
If there's a proper toggle mechanism inside the switch it will be in either on or off state. An in-between state (contacts just (not) touching) can cause arcing, this will burn the contacts of the switch. What happens to the load depends on the load. An ordinary tungsten lightbulb or heating element (toaster, oven, water cooker) usually don't care. Anything with a something inductive inside like motors, fluorescent tubes (they have an inductive ballast) and anything with electronics inside will not like this either. Eventually they will break.

Just get good quality switches and then it should not be a problem.