You're liable to cycle out a good round you might need later.
I've heard of two situations along these lines.
A friend of mine is married to a police officer who was involved in a shooting. The officer's partner was carrying a shotgun with a chambered round, but when the attacker presented with a firearm, the officer racked the live round out of the chamber instead of pulling the trigger. In the meantime, the attacker shot him in the neck. Fortunately the injury was only a graze wound, and his partner responded with a single shot that ended the confrontation.
The second incident involved a bear attack in Alaska. One of the defenders was carrying a shotgun, and couldn't remember if a round was chambered or not. Because he couldn't figure out what condition the gun was in, he panicked, threw the gun at the bear and jumped in the river. One of his fishing companions had a bit more presence of mind and was able to down the bear with his handgun.