Because most states (except Maine) use first-past-the post polling, this really only takes votes away from one of the major parties, and most likely from the more moderate one you might prefer. The GP is right, until RCV exists (or other plural voting option) it’s just throwing away your vote, or worse helping the more extreme candidate you don’t like.

Edit: to be clear, first work inside your state to pass some form of plural voting, then support 3rd party candidates.