I have a site I am working on, where I have a few custom taxonomies - Type of Food, Type of Accomodation etc, with terms in each.
I also have a set of categories like Food & Drink (sub categories Bar, Restaurant, etc), which I use as the main organisational bit of the site's contents.
Ideally I'd like to somehow link my custom taxonomies to the categories - so when I am in the Food & Drink category I can get the Type of Food taxonomy, maybe as a widget on the sidebar. However when in other categories that taxonomy doesn't show up, but other relevant ones do.
I hope this make sense - it could be just one of those times I am overcomplicating it, but I can see this as being quite useful.