[irinotecan]

Well, since they wanted the ability to build any previous version of PostGreSQL from the version history, for them I'd say it was quite valuable. I would suspect this is the main driving issue for whether or not to preserve commit history when porting to a new VCS; for some developers they may almost never go back and rebuild old versions from scratch. For OSS developers though I can see that they would need to preserve "full disclosure" and if possible make every commit history available for others, if not the devs themselves, to analyze.

[selenamarie]

Yes, PostgreSQL supports major version releases for five years so preserving the full history is critically important. Developers regularly back-patched bugfixes as far back as 7.4 (released November 2003, and just EOL'd this month).