I think with a teat it is easier for them to let the milk sit in their mouth and around their teeth especially if they fall asleep with the bottle. Whereas with a cup they can't hold it in their mouth while sleeping so milk doesn't sit there ITMS.

My friends son had to have his baby teeth pulled when he was 3 and the dentist had told them it was because of the teat and falling asleep drinking.

I'd imagine if you were good with removing the bottle when they fall sleep and ensuring their teeth are brushed well, it would be ok.