We have had some instances where modifying an upstream charm has been handy/necessary, either for custom functionality or pending upstream fixes where we didn’t want to track edge in production.
I think if you look at the community results in the charm store or on GitHub for some of the more popular charms, that it shows there are a lot of people publishing the same charms in their private namespace. Of course I am not sure how many actually have changed much (though that would be interesting in a way of who-forked-who sense).
For us a better CI/UX story for the charm store in combination of the comments of @erik-lonroth above about promoting/testing/quality/rating/ranking which most of the other popular cfg mgmt tools have. I hope these things will be included in the new store.