Great question.
At LAX, many airport screeners will not let you install locks after they x-ray your bag. Some of the mesh covers are a joke and fall apart easily. They also will get snaged on the luggage transfer belts. At many airports the airlines will give you a giant plastic bag. Be sure they place a duplicate luggage tag on your bag as well as on the plastic bag in case it gets ripped off. Call your airlines directly. When you claim your luggage, always check your bag, locks and zippers. If concerned, open pockets and check contents before you go through customs. The only belongings I have in my checked baggage is clothing. All valuables are in my carry-on. In Los Angeles, many baggage handlers have been arrested for stealing valuables from passenger’s luggage as they load the carts and the planes. It is amazing how many stupid people pack cash and expensive jewelry/watches in their checked baggage. Treate your valuables like your baby – Never leave them out of your site or leave them at home with a baby sitter.

On trains, people sometimes use small padlocks to close zippered pockets. May work well to keep people from putting stuff in, but are essentially useless to protect against taking stuff out. Same goes for putting a chain lock around your whole pack – someone can always cut a strap. However, the added effort of having to cut your bag may keep some people away. Maybe.

benaphex,
last year when travelling overseas, I used locks on my backpack only that I bought from Kathmandu. These were the combination ones. I used them everywhere I went and found it quite effective.

I’m travelling again shortly and will be doing the same. I doubt anyone will bother going through your luggage if your contents are locked up over someone else who hasn’t bothered!

I was considering security mesh for the same reason as you (all the goings on here in Australia) but I think it’s really more a hassle than anything else, plus I’m sure the more heavy duty ones will eat into the weight of your bags!

Hope this helps!