Yeah, I've known designers like that too. They're generally losers. Working with fixed-size elements is one thing, and being responsible for some limited part of a design is another, and those are both perfectly reasonable. I'm talking about managers and business owners who ask you to design a whole site, then draw up some design with their 13-year-old as a "project" and think it's pretty neat, so that should be your design. You can create something with it, but it's hobbled by the lack of thought that went into the initial design. And no matter how much you try to convince them and show them tests that changing certain parts would be good, they're emotionally attached to their idea.

I'm sure this isn't how you work, so it's hard to imagine other people are like that, but just like there are some unreasonable contractors who ignore your specs, there are unreasonable clients who can't hold up their part of the professional relationship.