Why should I take her to a dentist now?

My daughter just turned 3 and the pediatrician recommended taking her to a pediatric dentist. He referred me to one and I think he just gets money from referring. I called to make an appt. and the said they would do x-rays. I'm really worried about the radiation. I was listening to public radio and they said that cancer is developed from a cumulative effect from x-rays sometimes. She is so little, and she doesn't act like she has a cavity. To me, this just seems like a cash cow for the dentist. I don't think I had a cleaning until I was like 8 years old. Why is this so necessary? Also, they said the cleaning would take 30-40 minutes and I really don't think a 3 year old will sit there that long for her teeth to be worked on. The teeth are going to fall out anyway. Am I being a bad mom by not taking her to the dentist right now. Honestly, I was hoping they could just look at her mouth for cavities. They said the only way they could tell was x-rays. Anyway, I don't know what to do!!! Thanks for any input.