I used to use it on motorbikes all the time, for any fastener that might vibrate loose and/or had awkward access - luggage racks, handlebar mounts, crash bars and aftermarket frame protection always. I don't use it often on a bicycle, mainly because most things are easy to get at and check. I tend to use it on carrier racks and mudguard stays, which IME tend to vibrate loose more than anything else.
Used properly, threadlock doesn't stop anyone dismantling parts. It just means that the thread still offers friction even after the initial slackening of a nut or bolt. Taking it apart, you have to battle it off all the way, not just loosen and spin it off by hand. But it does mean that although a fastener might work loose, it won't actually vibrate off before it is noticed. I don't understand the chemistry, but it seems that it cures in the absence of oxygen. In the container there is always a large air space which ensures that it stays liquid, but once on a thread and tightened up, oxygen is excluded and the threadlock cures. Clever stuff.
