Hi Tony
This is one of those things that's hard to advise on, as you don't actually know if it's working or not. I've always had one, a blade type, and have never had a twitch from the caravan. But then again I've got to the end of the journey a couple of times and found that the blade has jumped out of it's slider, so wasn't actually doing anything, and again it didn't feel any different. But how many times it's actually prevented a snake or sway I don't know, there's no way to tell. Good loading is the best way to prevent a snake, but then again many properly loaded 'vans have been caught out by a sudden gust of wind or a high sided vehicle pressure wave.
So I would say that's it's better to err on the side of safety. After all what's cheaper, a stabiliser or a written of outfit? Not to mention the upset and embarrassment of seeing your caravan contents spread across the M4 during rush hour, causing the closure of the motorway.