This makes implementing a cache with futures much easier. With the current architecture a cache that uses Shareds cannot honor the "drop as cancellation" feature of futures. And the cache is forced to either poll futures itself to ensure they're polled to completion or to leave half-polled futures dangling inside it (potentially consuming resources like sockets or database connections). WeakShared is to Shared as Weak is to Arc. If any copy of the underlying Shared exists that has not be dropped or polled to completion the WeakShared can be upgraded to it. This makes it possible to construct a cache that does not entrain futures, thus honoring the "drop as cancellation" feature.