An interview is a two-way street. Presuming you're the candidate, the organisation is selling itself as much as you are you. The key difference is that there tend to be more candidates than positions --- the employer has a superior BATNA (best alternative to negotiated agreement).

That said, I've had several interviews I'd concluded I wasn't interested quite early in the process. In one case I wrapped relatively quickly, and immediately told the recruiter I was not in the least interested. In another, the situation wasn't hostile, but was so bizzare I continued the interview simply to try to understand what the heck was going on.

It would be extraordinarily rare for an interview to pose a direct threat, so continuing with a viewpoint that the experience is simply practice doesn't hurt. I've also had other staff who'd interviewed me and then left that position contact me with other offers, so from a networking perspective, the experience can still be useful.

There were other opportunities I should have passed on but did not. Having additional options is extraordinarily useful. Those are among my regrets.