I cant figure out a clear answer, which clarifies whether tag clouds are spam or not, especialy major blogging tools tag cloads which return a duplicated content of the orginal posts which its related to. ( here.. you have to think about the duplicated content penalty)
But... on the other hand.. using it will have a good impact on your search results (no one doubts that) !.
Simply because it returns a page with duplicated content !
it might be confusing.. but yes the page has a different name.. please check out this : [google.com...] look at ----> Don't create multiple pages, subdomains, or domains with substantially duplicate content.
and thats what tag clouds is about... it creates multiple pages with substantially duplicated content.
For one, they probably would not be duplicate content because the tag page would have content specific to that tag and another tag page would have content specific to that page...so the content would be different than any other page on the site. Yes, the sub-content would be duplicated from another category page but the total page content would not be a duplicate.
I think that "out of the box" tag pages are somewhat low in value.
But if you're a little creative and customize them they could be content rich, useful pages.
I would also urge you to think twice before taking everything you read in Google's webmaster guidelines as gospel. Consider the source...they have their own motives.