Time-in-the-future transactions are non-standard (as of 0.8? I can never remember when things happened...)
Why: because there is a fill-up-memory denial of service attack, and it really isn't reasonable to expect the entire network to store your timelocked transactions "forever".
Even in the past, the statement "unspendable by the sender because of replacement not being implemented" was not true. Wait long enough and only a subset of the network will have the timelocked transaction (because new nodes, old nodes restarting, etc). Broadcast a double-spending version without a timelock and it will get mined fairly soon.