I disagree. The question is actually a basic question to see if someone understands discrete math. It's a cornerstone formula, IMO.

Now, if it's the only question that you ask, then it's a problem. If it's related to the job, though, then it may be more important than you imagine.

One final note: just because someone might be the right hire in the future doesn't mean that they are the right hire for the present. If the person does not have the needed skills or experience, then they are the wrong hire. In the future, after having acquired the necessary skills and experience, then they may be the perfect person for the job.