We run a WordPress blog with almost 94.000 post tags, spread over 5000 posts, and only 20.000 of these post tags are actually attached to more than one post, hence 74.000 of the total number of tags are used for one post exclusively.
My pal who produced most of these tags, is firmly convinced that these tags improve our site's SEO, which I absolutely doubt. He creates 20-30 new tags per post and hardly uses existing tags.
I mean, even if all of these tag archives are indexed by Google, I cannot imagine that it really helps with Google ranking when having >70.000 one-post-archives. If you stop indexing taxonomies archives I think it becomes irrelevant at all anyway.
Can you prove me if I'm wrong? It really has become a big problem since all plugins result in a blank page when trying to filter by tags. I guess the bulk of tags is far to big right now.