I've worked with all kinds of people. Some people turn out to be very opinionated. Some people constantly call back with "just one more tiny change" each time they've gotten any type of feedback from others. Some people want to use neon green even if neon green is absolutely the wrong color to use, simply because neon green is their favorite color. Get my drift?
There are two ends of the spectrum here: people who are driven by others' opinions — they are swayed by any feedback anybody gives; and people who are driven by personal preference — they want what they want, regardless of what anybody says. Am I complaining? Nah. But sometimes, every so often, if I'm having a particularly rough day/week/project, I pull the following video out of the archives to give myself a bit of comic relief. If you've never seen it, go ahead!
We designers don't know everything, and we welcome input (remember, creativity is only enhanced by collaboration), and we want our clients to be satisfied. We understand that our clients know their industry better than we will. However, we have done this once or twice before, and a good designer will have done adequate market research as well. Sometimes, it's good to trust our professional judgment, intuition, and past experiences! :)
Ultimately, we are paid to do what the client want, so if they are determined to have a neon green project, that's what they'll get. But don't say I didn't warn you when your communicative value is diluted (because people are shielding their eyes to avoid burning their retinas). If your message is lost, your project is not successful. Let's always remember it's about smart design.