Ahh, I am glad you asked. I actually have been lucky enough to work at two small spacecraft companies. I am not an aerospace engineer but I talk to people who are and interact with tons of x-nasa people. Also, my weirdo comments aren't my own. Checkout the omega tau series, especially http://omegataupodcast.net/218-a-life-in-apollo/
People telling you things is evidence.
Actually,that's a good point! People telling me things isn't exactly evidence. I wonder what pure evidence would look like?
No, it is exactly evidence. But, as you seem to be aware, each kind or instance of evidence should be weighed differently, i.e. stronger evidence should sway your beliefs more strongly.
People telling you things may not be particularly strong evidence, but you should still probably update your beliefs at least a little.