I lock the zipper pulls and secure them to the handle of the case. This prevents a baggage thief from resealing the zipper, making it a slightly less attractive target.

I've also got a couple of very old (> 50 years!) heavy duffle bags with metal zippers. You can't use the ballpoint pen/reseal trick on metal zippers.

It simply isn't always feasible to take everything of value as carry-on, particularly when you're talking about gear that can't be easily replaced, if at all, at your destination.

Example: the couple who arrived in Africa to hike Kilimanjaro. Their unlocked bag had contained well-broken-in hiking boots, trekking poles, one Goretex shell, batteries and OTC meds and toiletries, all of which were gone. The thieves even pulled the battery from their travel alarm.

The batteries were replaceable; a poor substitute for the hiking poles could be found, but the boots and the shell couldn't be replaced.