Well it can't be 100% but it does reduce spite theft and vandalism. In the end a thief is a thief, is a thief - no getting around it, but I tip and remove stuff so I have never had it happen by a valet.
I have had other theft incidents though, I mean in a city it is silly to not expect it so all you can do is limit loss.
Car keyed, yes.
Windows smashed and purse stolen, yes.
Bike stolen as a kid, yes.
Jewelry stolen, yes.
Blaupunkt radio stolen, yes.
I get everyone is different but the change from my car wouldn't even raise an eyebrow for me, I'd be annoyed I forgot to empty it to be honest but that might be a city mouse country mouse thing.
Only way to completely prevent theft is to never have stuff to steal, so to be Yoda.