This would allow an application to not persist its state but just rely on tx replay on startup to restore the last state.

Typical use-case would be to re-initialize the state after code changes / upgrades. It seems a good idea to do a replay vs. letting the app persist the state itself (do snapshots etc), as it will force the upgraded application to demonstrate that it can gracefully process all old txs.

Not knowing the internals of what Tendermint stores, it might be possible to determine the point from where to replay based on the application hash returned in the first commit() after the TMSP connection is etablished.. Alternatively something simple as just passing a commandline parameter which will force this behaviour on initial connect after startup.