You can definitely empty them and put them back, you just have to replace them when they tear otherwise all it's going to do is suck the dust up off your floor and blow it around inside your vacuum, contaminate your vacuum fan and motor and eventually render the vacuum cleaner inoperable and possibly flammable.
Just to clarify something... having a full bag doesn't mean it'll explode in flames or anything like that (normally at least) - it just means your vacuum won't vacuum... there's no place for the dirt to go so the vacuum just lifts the dirt and spreads it across your floor.