Beastie, like a famous Dutch soccer coach said: "Each advantage has it's disadvantage"
The advantage of moderation is that the already joined members (the majoritiy) won't ever see any spam, while the newly joined members (a minority) experience a small lag before their post becomes visible.
Maybe CarpetSmoker, who is more familiar with the Vbulletin software,then me, could explain which other possible options would be.