I disagree. It is very difficult to spam the site when creating tags (giving only the label). You only have 30 characters and it is quite difficult to get anything useful in there. Also, it will be noticed quite fast since the question gets enough eyes usually.
Tag wiki on the other hand are not that visible and very prone to spam (to a blog, some external source, or an 'advertisement' tag wiki). It won't get eyes automatically. Hence, it is important the tag wiki edits get reviewed.
There is something to say for tag creation with mandatory excerpt or wiki, but you have to keep in mind that the user creating the tag isn't necessarily someone with enough knowledge to write a good excerpt or wiki.