It’s not a good tool if one party is likely, but not guaranteed, to win without your vote, but is much worse than the other. You should only spoil your ballot if your constituency is has a large enough majority that your vote won’t matter at all, or none of the parties are less bad than the others.
The first instance is realistically the only case in which it would really matter that you spoiled your ballot, though. In the second example of when you might spoil your vote, it wouldn’t really matter at all, precisely because they have a large enough majority.