There are bad doc/dentists. There are bad everywhere.
I think this point is missed every once in a while. People tend to look at an MD/DDS as being able to do no wrong, but, like everyone else, half of doctors are below average. You can use what little signal you have to determine if a doctor is good: what school they attended, how many years they've worked, how many times they've done a certain procedure, etc.
Those signals are well and good, but there are other signals that aren't expressed in any other way that these review sites: when you're leaning back in the dentist chair, will you be looking up at cobwebs? How does that reflect on the general hygiene of the tools going into your mouth? Is the office staff friendly to children or is it staffed with anti-breeders? Those subjective measurements aren't reflected in a simple list of facts.
For the doctors out there, if a someone is dumb enough not to be able to see past some random idiot posting a inflammatory or untrue review, maybe you don't want them as a patient.