FF & LN
This may come as surprise but Jim59 makes a totally valid point. A person's character may be defamed if a malicious statement is conveyed to one or more people (hence the use of PM makes no difference) by publication (libel) or the spoken word (slander). In law inferring a professional person cannot carry out their occupation competently is generally accepted as malicious (and therefore defamatory) unless proven to be true. The only person(s) who could so comment on a doctor's competence would be one of his/her peers or a regulatory authority. Certainly not a pilot. Ergo if I commented in negative way about a doctor's competence and my comments could be proven to be made by me (and without substance) I would be well and truly screwed.
There is a mass of defamation taking place all over the internet on a daily basis. Just because it has become virtually normal does not make it right and, more importantly, every now and again, somebody steps over the mark and ends up in court.
The defence "everbody's doing it" will carry very little chance of success!