What did your doctor say about having an elective 3D or 4D ultrasound

I had my regular, medical ultrasound at 20 weeks. There is a place in my town - not a doctor's office - that has 3D and 4D sonogram equipment. They require a doctor's authorization before doing the ultrasound, but when I asked my doctor to sign it, he seemed rather negative about it. He did sign it, but said that he cautioned against it because these are non-medical ultrasounds performed for keepsake purposes, the staff performing the test are not qualified as medical personnel, and that unless there is a medical reason to do one of these he would caution against it because the risks of having extra ultrasounds is unknown. I still want to do one, but now am a bit freaked out about it. Can others share what their doctors said about elective ultrasounds?