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Prototypes are worse than pointless, they give the motivated and ignorant the belief that work has been done, that things exist. Write a sentence that describes what's wanted. Start it for real, iterate, test, test against goals and deliver, repeat.

Prototypes are the most powerfully tool I know of for increasing the productivity of conversations about what software should do and how it should work They're tools for thinking

Paper prototypes are not always useful, I'd say. They are nice for quickly evaluating ideas for UI structures. For evaluating e.g., acceptable latency of a UI, one would want to build a high-fidelity prototype. (BTW: It seems like everyone here is equating "prototype" to "app with user interface").

A problem you are rightfully mentioning IMHO is that UI prototypes usually look more finished than they are, which may mislead users/stakeholders - and maybe subconsciously also developers. Stakeholders may underestimate the time needed for completion, and users might be less willing to criticize →

aspects of the UI because they think that the system is "nearly finished". Also, a high-fidelity UI prototype invites bikeshedding - people will be more inclined to discuss, e.g., the color of buttons than the information architecture. Usability experts have been aware of these problems for decades.