There's still an unquantified pile of hidden results that stem from studies that never register with ClinicalTrials.gov in the first place. (E.g. studies done outside of the US.)How many studies done outside the US are not registered at ClinicalTrials.gov? If you intend to use your post-phase 1 trial for FDA approval, you have to register within 21 days of signing up your first person. Since the US is where drug and medical device companies make all their profits, I can't imagine why you would spend the money to do a trial just to have to rerun the trial later for FDA approval.
Plus if you ever intend on publishing in any of the major journals, you have to register before your trial starts or you'll be ineligible to publish (including phase I trials).