The first point to make is that 'dry' steam cleaners are NOT the same as soil extraction carpet cleaners (also sometimes mistakenly referred to as steam cleaners) and, indeed, 'dry' steam cleaners dont actually produce dry steam! The reason they are referred to like this is because compared to a traditional steam cleaner (ie a hot water pressure washer you would wash your car with at a garage) they use far less water (typically up to 10 litres an hour as opposed to 600 litres an hour).
There is a great deal of confusion about dry steam cleaners and what they can and cant clean which has developed because certain manufacturers do claim they can clean carpets. So it is a good question - do they or dont they?
The first point to clarify is that virtually all dry steam cleaners work on the same principle at similar temperatures. What distinguishes them is the steam pressure, the capacity of water they hold, the power and quality of the heating element, the control of the output and the range and quality of the accessories.
Next, you need to understand how these machines work to appreciate the difference between cleaning hard non-porous surfaces (tiles, bathroom fittings, vinyl floors, kitchen equipment etc for which they ARE very good at) and typical carpet fibres. The fundamental difference is that on hard surfaces the dirt doesn't penetrate the surface. So, once the steam has loosened the dirt, it is easy to remove it with the terry towelling cloths as the steam condenses (reverts from steam to water). However, with carpets the dirt penetrates deep down between the fibres and, although the steam may well loosen the surface soiling, these machines don't have the facility to extract this embedded dirt from deep within the pile.
Proper carpet cleaning machines also use a much larger volume of water to flush the dirt out which is then extracted using a powerful vacuum. In effect what happens with a steam cleaner is that the dirt on the surface is removed and short-term the carpet does look cleaner. However the dirt left in the carpet soon 'wicks' back up the fibres and the carpet is dirty again. You may have heard people say that carpets seem to get dirtier quicker once you've cleaned them but this almost always indicates that they were never cleaned properly in the first place.
Before we get too negative about steam cleaners and carpets there are certain tasks that we should highlight. Because of the extreme heat dry steam cleaners are very good at killing dust mites within carpets and bedding and can reduce household allergens. They are also good at dealing with smaller stains and spills and are good for killing bacteria and combating odours. Curtains and clothing can also be steamed to remove creases and malodours. However, this doesnt qualify them as being good carpet cleaners because they simply cant 'deep clean' - you wont find a single professional carpet cleaner who uses a dry steam cleaner to clean carpets!
In summary.
Dry steam cleaners are intended primarily for hard non-porous surfaces.
They are good for 'freshening carpets' but not 'deep' cleaning.
They are good at killing dust mites and combating odours.
If you need to 'deep clean' carpets buy a dedicated carpet cleaning machine.
As with anything else in life - you get what you pay for!