Very little spam seems to be totally automated, but most spammers likely use some kind of tool to help them spam faster. There are many weaknesses in the different wiki and blog implementations and even when improved many people don't upgrade. This leaves you open to attacks by spammers using automated spamming programs.
Often spammers (automated or not) won't even look at the page they have spammed so they will never know their spam was blocked. That is why even if you have antispam prevention or don't allow external URLs on your wiki spammers aren't going to go away.
We have seen one example of a SpamBot going nuts due to antispam features blocking the URLs it adds. It kept editing the same page over 300 times. See SpamBlockLoop.