You really think it's easier to stick a pen into a zipper, open it, and then try to close it when the pulls are locked than to open a twist tie?
Pickpockets are opportunists. They look for the easiest mark, the easiest bag to get into. If you make it difficult for them, they're going to move on the someone easier. They don't see bags as "challenges."
Sorry, but locks have proven to deter theft. It just takes too long in public to try to get into them. Twist ties will also deter but can be opened much faster than a locked case. Nothing is 100% fool proof.
How much experience do you have with this? I'm a certified travel goods specialist who has had training in luggage theft deterrence?
I've been to seminars given by former professional pickpockets and thieves. The one thing they stress is the harder it is to steal something from you, the less likely you'll be a victim of theft.
Does it guarantee that you will never be a victim...no. But by being observant and making it more difficult, the less likely your chances of being a victim. There are just too many unsecured bags around for thieves to choose from that they will tend to go towards those than ones who have added an extra obstacle.
But you can believe whatever you want.