It seems clear that this rule has been put into place as a step toward reining in the copying of test questions for the purpose of distributing them to people. Supposedly, cheating is one of the reasons for the upward drift of the scores associated with quant percentiles.
Also, it seems possible that since students no longer have to show cancelled scores, there has been a surge in people's taking the test many times, and there is some reason that GMAC prefers to shut that down. I can say that I have seen people taking tests almost on impulse, without much chance of hitting their goals.
Meanwhile, given that there is some degree of randomness in scores, with the ability to take the test without reporting scores, a person could take the test many times and report only the the best score achieved, one that might be an outlier.