That tab is just there to hold it in place. If you have a large card with a heavy cooling solution on it then gravity may cause the card to slip out. I'm sure you could figure out a way to brace it somehow if that was a problem.
If the tab is broken then there is no adding it back. If it just needs to pop back into place I'd go ahead and do it. Take care not to push down on the motherboard too hard so you don't crack it.