As one who works with analytics databases and ETL (extract-transform-load) processes a great deal, immutability of data stores is an incredibly valuable property. Maybe append-only does not make sense in operational databases all the time, but for non-real-time analytics, it makes a huge amount of sense. In my case, operational data is queried, optimized for storage space and quick loading, and cached to disk. Because it is an analytics database used for longer-term analysis and planning, daily queries of operational data are sufficient in many cases. Operational workload is not even a consideration. The ETL process also allows for "updating" records in the "T" (transform) part. Updates to operational data are not even necessary, and often impossible, so correcting and enhancing the data for decision making is a huge win for clients. Issues similar to "compaction time" can still occur, but an ETL approach allows for many clean ways of controlling the process and avoiding those failure scenarios.