Ive considered this in the past and I haven’t adopted it up to now because of the following reasons.
Forums are built around the formative members. Paid forum posters will come and go whilst your regulars will stay. Its important to tempt people who have an interest in your forum as they will help the site survive its darkest hours.
Ive always been concerned about the quality of the contributions by paid posters and for that reason I think Ill always consider PPC before I resort to using forum posters. With PPC the users you attract to your site will be more targeted especially if you target keyphrases like webmaster chat or webmaster forum.
Paid posters aren’t likely to refer a friend. Even if they do, they’ll probably be looking for payment too.
If natural members of your forum find out you’ve been hiring posters it might cause some unrest. Why should anyone else get paid when they are contributing information for free?
Paid forum posters cheapens the idea of a community. Much like you don’t pay your friends to hang around with you, you should avoid doing the same with forum posters. Instead concentrating on building relationships on your board.
I still have no real interest in hiring posters. I doubt that’ll change any time in the future, unless I become really desperate.