Is there any situation in which it is better for a Police officer to have a firearm than an accurate and quick-acting tranquilizer gun?

We might have to spend money developing a gun that could fire a tranquilizer dart as accurately as a bullet and that it would have a similarly instant effect, but it seems to me that this would save countless lives, preserve the mental stability of our Police force (since they would never have to shoot anyone) and make the communities they police see them in a better light.