This topic always creates a heated discussion. 15 years in the industry and I'll give you my take on it. This system has a better insulating value than just a small amount of insulation on the shell of the tub. This system is supposed to create an air tight area and the equipment then heats the air and the air becomes the insulating value. One problem is that equipment doesn't last as long when exposed to high levels of heat over time.
The reason manufacturers do this, in my opinion, is that they have a problem with leaks and it is a great way to cut costs out of the manufacturing process.
The most energy eff. way to build any product is to use full foam insulation.
Two recommendations, look clossely at the warranty and all of the fine print written and then test soak the exact spa that you are considering.
