I'm running 10.8.2 and crontab entries stick around when I reboot.

Do you know where you learned about crontab entries being mandatorily deleted? Perhaps it was introduced with 10.6 but reinstated in 10.7/10.8?

Personally I can't see crontab entries causing any problems, and the technology is so well established in Unix culture that removing it would alienate a lot of people.

It was an experience I documented here:

http://hintsforums.macworld.com/showthread.php?t=101270

This article...

http://alvinalexander.com/mac-os-x/mac-osx-startup-crontab-launchd-jobs

... explains it all.

Quote: "The cron utility is launched by launchd(8) when it sees the existence of /etc/crontab or files in /usr/lib/cron/tabs."

launchd has indeed superseded crontab, but crontab has essentially been turned into a launchd service! Did somebody say "legacy"?

As I say on my blog posting linked to in the article, what I outline is better done as a launchd service (and the web link above explains how), but it's just a lot simpler to setup and run it as a crontab event, and it's just as effective.