To be honest, I'm not sure how useful the tag cloud is on Flickr. Photos aren't the best sort of data here.

As you pointed out, tags are very good at tracking trends, so to find the best use of a cloud you have to think about situations where trends need to be spotted quickly.

An example would be a database of customer complaints, each complaint tagged appropriately to indicate something about its nature. A tag cloud may help support reps to spot trends they otherwise might have missed as tags amass and grow in size.