You can get value from a naysayer or cynic by staying objective and positive. (Yes, it can be tough).
Occasionally, naysayers or cynics might point out a valid hurdle or challenge you haven't seen (even if they present it like an ass).
With their help, if you can remain objective (and keep your ego in check ... be no ego, right?), you'll have a better chance of getting something valuable from the interaction.
Stay objective. Ask questions. Listen. Get value.