Responding to my own question, here's what I've figured out:
I was incorrect about the "insert was silently lost" problem. It was actually a partially processed set of deletes (in separate transactions). Apologies.
The save_internal is mainly for the situation where the destination ASE server crashes and loses a little bit of recent data. In that scenario, you can boot the repserver with the problem DSI suspended, run "sysadmin restore_dsi_saved_segments" and old transactions will be reapplied, with transactions that are older than the rs_lastcommit date being ignored