I would agree that immersion or tub area rug cleaning is going to get a rug cleaner than a typical in home wand cleaning done by rug cleaning professional. There is no doubt that a lot more soil is removed in the immersion process. At the same time most rugs do not need to go through that deep of a cleaning. In Tampa Fl we have a real problem with sand. It gets into your home and easily finds it's way into soft surfaces, like upholstery, carpet, and area rugs. Although sand is a problem most rugs do not need to be tub cleaned to remove the majority of the sand.
As a professional area rug cleaner I understand that very heavily soiled area rugs tub cleaning is the best solution. At the same time removing a rug from the home can be difficult or even impossible in some situations. There have been many times that I've had to clean oriental rugs that had a pool table placed on top of them. In most cases an in home rug cleaning will be sufficient to remove the majority of the soil.
Another case for immersion cleaning is that you wont have a sopping wet rug on the floor ruining your hard wood. I have cleaned hundreds of rugs on hard wood floors. You just have to take the necessary steps to make sure the back of the rug does not get wet and that any over spray is toweled up.
In most cases I prefer to do area rug cleaning back at the shop but that doesn't mean you can't get a quality rug cleaning in your home.