Main trouble is, BS, when such ‘consent’ also extends, in the small print, to “trusted partners” or some such twaddle. That means they can sell on your data to all and sundry.
I have no idea whether my dentist’s form had such a clause, as I simply declined. But data is worth money, and they are in business to make money, so I’d be surprised if they did not want to sell it onwards.
Even if is just local Dentist though, acting for themselves, I actually do not want them calling/texting me just because a checkup is due. They have my mobile number, which I supplied so that they can contact me if (say) the dentist has called in sick, to save me a wasted journey, but I don’t want them using it any other time.
I’ll have to wait and see if GDPR puts an end to the annoying reminders. I do hope so.