Online use of the term "moderator" has its roots in UseNet discussions, where technical provisions were made early on for a person to review posts prior to distribution. This predated the spam epidemic, and was initially used to prevent duplicate and offtopic postings. Since posts were not distributed prior to "approval," this early system was a form of PreemptiveModeration.
Preemptive moderation has become increasingly rare because the delays it introduces stifle active discussion. Instead, most online communities (including all wikis) distribute content first, then attempt to remove it later if it is deemed inappropriate. In some cases a hybrid system is used where only contributions that fail to meet certain criteria are held for moderation. For instance, the ScoopEngine provides both a submission queue and an edit queue to vet articles before posting. The difficulty is that inappropriate material, once distributed, may create offense in the FirstReading or waste time; retroactive removal of content cannot reclaim what is lost.
At the present time, wiki technology is not pervasive enough to attract spam or, for the most part, persistent attempts at vandalism. With time, this may change, and some sort of preemptive moderation technique may be necessary to retain sanity.
Pre-emptive moderation may still be useful as a ShieldsUp measure: in case of severe attack, switch to pre-emptive mode, and get ready to pay your moderators overtime. BBCi (such as HTwoGTwo) has this option, though it has not yet been used. There is a master switch in the database that will switch the entire site over to pre-moderation.
See also CommunityWiki:PreemptiveModeration