I have always thought that this is a brave thing to do. There is the very definite risk that some snotty nosed and bored child will decide to pick a bunch of, "pretty flowers," (aka the owners pride and joy from the foothills of the Himalayas), or that a geriatric plant thief will dig up some small alpine or similar to improve his or her more modest plot.
Some of the plant burglars are rather more subtle. These are the ones who perhaps have a modicum of gardening ability and are willing to put a bit of effort into their venture. Rather than stealing whole plants they will purloin cuttings for themselves. The best prepared will even have small plastic bags and a supply of green ties hidden about their person for use during the heist. All of that said I didn't see any nefarious goings on at the garden I visited on Sunday afternoon. Maybe the owner was just lucky or maybe the fact that the gate money was in aid of a Church roof provided a protective benediction to the day.