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I fasten the two zips of my one together with a keyring. Not the whole thing, just the actual 'ring' part. Secure, but needs no tools to get it off. Easy.
If its easy to get off then why bother - seems to defeat the whole object of you supposedly securing it in the first place 
Because they're fiddly and take time to undo, without being impossible (well if you're wearing gloves they are impossible). I've used them for years. They make the lock into a 'not worth the bother' deterrent.