Besides the points mentioned above about average scores, from my experience, taking it just once also sends a signal to the Admissions Committee about drive and desire.

I helped evaluate candidates in my MBA program as an interviewer, and got to see more about what the Ad. Com. thought process looked like. Unless someone knocked the GMAT out of the park on their first try (e.g. 750+) then they would would look down on those that took it just once and didn't try again. Especially if you have as much time as you do to study and retake. They often view a 1-timer as lacking the drive to improve themselves. They might call you out on it in your interview and ask why only once, and "I thought my score was good enough for this program" isn't an impressive answer.

Now at a certain point (taking it 4+ times) it doesn't look good to keep trying, but I really think taking it twice should be the plan for everyone. (Just make sure you study hard and get a higher score your next time.)