Usually when I import the photos off a card, I’ll recycle the card right away. That’s assuming that I’ve imported and backed up immediately, or imported to the Drobo at a minimum. But sometimes if I’m on the road and I’m just not shooting that much, I may want to import pictures long before the card is full—and would like to leave the existing photos on the card as an extra backup, at least until I’m back home.
Aperture has a nice “Do not import duplicates” feature in the Import dialog, but as you may have noticed, if you rename Masters on import, then this feature no longer works. It relies on matching file names to detect duplicates—so once you’ve renamed your Masters, there’s no name to match.