Ofcourse it is. It just means absolutely nothing. Nightwish is metal. Opeth is metal. Cannibal Corpse is metal. Any of those bands can claim to be metal, yet none of them sound even remotely like eachother. It's about as useless as saying you're in a 'pop' band.
And while people can argue about subgenres until kingdom come, usually it's not *that* hard to atleast pigeonhole a band into one or more ballpark subgenres. If you claim that your band is power metal then no one is going to argue it's death metal unless either you or the other person have absolutely no clue what they are talking about and are infact just plain wrong. There's some genres that can overlap, like the endless debate whether certain bands are powermetal or heavy metal, but if the difference is so little that no one can agree on it then does it even really matter?