We had a similar system like that before yes, and it worked fine. There was like no spam at all (or low quality posts), except for the ones made by the "normal" users, which always happen.
But it was quite some work, for every one of the staff team, as new members were joining constantly at a pretty high rate, and all of them wanted to be approved asap, which wasn't possible to handle on the long run. So posting activity went down and that is what a forum is for.