IMO it would prevent a lot of the nastyness that exists, and a lot of trolling.
I personally think that someone who signs up purely to cause trouble, is generally just borde. If they weren?t allowed to post straight away they would forget about posting here and would go somewhere else. As for people who need urgent advice, if they?re genuine, signing up here would give them a chance to look through posts and probably find the answer to their question without having to post it for the hundredth time. And if the advice they need is that urgent,then it?s probably professional help they need anyway and not mn help iyswim.