Is it safe to keep the outer pockets of checked-in baggage unlocked?

Even a locked zipper will not prevent unsavory characters from putting contraband into your bag.

In all my travel (mostly North-American and Europe) nothing nefarious ever happened and most of my luggage have external pockets.

You could wrap your luggage in plastic wrap, a service that most airport offer.

Don't worry about it: smugglers planting drugs in the bags of complete strangers at the airport is largely a myth. It's a bad idea for the smugglers in many, many ways, starting at the planting end:

• Baggage handling areas are very busy places, there's little time to open up bags and lots of witnesses around. Bags "falling off the conveyor" because the X-ray guys spotted valuables? Sure, this actually has happened at some airports. Somebody taking a bag that's the right shape and going to the right destination, opening it up, planting drugs and putting it right back in fast enough that the victim can still catch their flight? That would be pretty challenging.
• Airports also have really, really extensive CCTV coverage, so anything they do would be recorded and could easily be pulled up by investigators.

The biggest problem, though, is how do the smugglers get their drugs back on arrival?

• They can't sneak the drugs out between the aircraft and baggage claim, because the whole point is to get them through Customs.
• Baggage claim is always before Customs, so no point stealing them back here either, and there's witnesses and cameras up the wazoo here too.
• After Customs while still at the airport seems like the least bad option, but how exactly do they get the bags off you if you're physically lugging them?
• And afterwards it's too late: they'll have no idea where the unwitting courier is going, how they'll get there, how many other people are present, etc etc.

And yes, people try to play the "I had no idea the drugs were there, guv'nor!" card all the time. Some of these people are willing couriers that are straight up lying, others were duped into carrying a package/suitcase without knowing its contents, still others had drugs hidden in their own bags by a boyfriend/relative/etc. But if there's any proven cases of drugs being snuck in by airport staff, I'd like to hear about it!