Designers also are typically not around to feel the repercussions of their designs, so they may think they are designing for other people when they are really going off of their own intuition. And let's face it, even those with the best design/marketing/usability/branding etc. intuition are often wrong (eg Steve Jobs/The Cube).

I tell my clients that results-oriented design is the way to go. Interfaces are never "done," and it pays to pay a designer to stick around and iterate after real-world testing is completed. Casual focus groups are a great way to start this process, often ensuring that you don't launch with a poor design out of the gate.