You have to put this in the context of all the other security measures. A hotel where I recently stayed has an elevator that requires swiping a key-card before use. There are cameras at the stairs, elevators and before the rooms. So, if something gets stolen, the thief will appear on camera. A log of all key-card swipes is kept. Now breaking into your room without a key-card is not easy, it would require a lot of time. This will be noticed on the security cameras during the attempt, so this won't happen.
Given this security situation, you're only going to be vulnerable to a hotel worker stealing your stuff unnoticed. The hotel worker gets in for cleaning, takes away something of value that you won't immediately miss, puts it in his/her pocket and gets out. If you only notice that this stuff is missing after a few days then it's not so clear who could have stolen it. It might have even have been accidentally lost on the streets, so you can't then use the key-card swipe data to narrow the culprit down to a single hotel worker.
The safe-locket is then a good measure to defend against this loophole. It forces you to collect valuable stuff, merely knowing that you did have possession of something a few hours ago, that this was present somewhere in your hotel room and is now not there anymore, will point to the hotel worker being the thief. So, even if the safe-locket can be easily opened, the hotel worker won't dare to do that. There is also no way for the hotel worker to physically remove the safe-locket and walk away with it, unnoticed. Taking a picture of your stuff before you close the safe-locket will make it this work even better,a you'll then have rigorous evidence to back up your case.
In a different context where you don't have much security in the hotel, a thief may well be able to steal items from a safe-locket and get away with it.