Yeah, this was always my understanding - time zones, DST participation and dates, etc all change more frequently than you'd expect, and time zone databases keep track of when things happen precisely for this reason. If that weren't happening, I think people would have found that many of their timestamps become obsolete / inaccurate relatively quickly.

That said, it does tie the stability of your datetimestamp to the stability of the tz database. I think that's a pretty good bet, but it's nice to be aware of where your abstractions can leak.