I thought that there were already numerous things that can't be advertised, despite being legal.
Cigarettes is the first one to come to mind but I'm sure there are others...
Despite the well-publicised downsides of payday loan companies they still seem to do ok out of it, implying that there's a need to be met.
If the adverts (or companies themselves) were to be banned then you're likely to see an increase in people using (completely unregulated) loansharks...