This would have at least two use cases that I can think of off the top of my head:

• Make it easier to track down users who consistently leave off certain tags from their posts, or barely bother tagging at all
• See what tags people are adding to your own posts, especially if there were tags you didn't know existed or weren't sure about (and of course make catching the occasional vandal easier too)

An inverse form would also be nice to have, to exclude self-edits and new uploads.

wat8548 said:

• See what tags people are adding to your own posts, especially if there were tags you didn't know existed or weren't sure about

I approve of this, it would be good to see what tags I'm missing out.

bump

+1 from me. I'd love to have this feature.

I'd also like to have this, it's useful to know what tags I'm often forgetting to add and it'll probably help people find tag vandalism on the posts they've made too.