The precise definition of life will not be the thing that will determine our opinion about possible extraterrestrial life when we come across it.

It's possible that extraterrestrial life will be nothing but a soup of RNA molecules. If we visit a planet while its life is still in the embryonic stages, we need to include that in our discourse of life in general. We need to have a word to represent what we are talking about when we talk about it. That's the only purpose any definition ever serves. If you want to go down the route of 'the definition of life is useless', you might as well just say 'all definitions are useless'.