To spam your wiki, spammers will first have to find you. All spammers use Google or other search engines to find spammable wikis. Some search for wiki-specific phrases like "Edit this page". The majority of the spammers that visited the chongqed.org wiki though, were searching for instances of other spam. Look at the referrer strings of the spammers on the SpamPage.
In essence, this means that spam on your wiki will attract more spam.
This explains why the NoIndexHistory measure has a spam reducing effect. If users keep your wiki clean, but search engines continue to index the spam in your history pages, then spammers will continue to find you this way.
It also explains some interesting effects we have observed during spam cleaning activities. A wiki can be rescued from a state of constant spam bombardment, if we just keep it clean long enough for google to see no spam.