Free plans suggest that your service is worth nothing, because you can give it away for free. That's not always acceptable, especially when dealing with larger B2B clients.
My point on the free plan is more like a trial where you get limited use, not giving away the entire thing for free. In interviewer.io's case I also want to be able to help smaller startups who might not be able to yet afford a paid plan at least use the product for the stage they are at.