What would be the reason for such a restriction? Obviously, they don't have to say you can't cast more than one spell in a round because spells already take at least a standard action in most cases. But before they defined swift actions, quickened spells were merely a free action. Thus, if they had not included a limitation, then one could theoretically cast as many quickened spells per round as he had prepared. I cannot see a rationale for including that rule otherwise. I really doubt the designers were worried about someone casting two quickened magic missiles. I think it makes far more sense if they were worried about someone casting two quickened magic missiles AND cone of cold. They obviously weren't worried about someone casting one quickened magic missile and cone of cold. So why would two quickened magic missiles bother them?