My use case for this feature:
I'm using Mongo as an analytic cache. Every period of time I get a fresh copy of an analytic and load that analytic into a "staging" collection and when the load is completed I rename the collection to the "active" collection name. I was hoping to shard those collections so I could use the horizontal scaling of Mongo to cache larger analytic output.
This pattern is very similar to how mongo's $out aggregation operation works if an existing collection exists.