I´m asking this because a possible attack would be to send an flooding number of orphan transactions, which will never be redeemed. Then clients would have to cache them, increasing storage requirements and reducing lookup time. This can lead to a resource-exhaustion attack.
If there is a limit on the size of the cache, then a DoS can be mounted to exhaust the limit. In both cases, a DoS attack can be mounted.
If there is no protection, adding a proof-of-work to each transaction would help to prevent such attack.