There is one simple reason why I get my carpet cleaned every 6-12 months, and that is to ensure that the home environment is as hygienic as it can be. Hard flooring is easy to clean with a mop, and an antibacterial cleaner. This is not the case when it comes to carpets. Due to the way that carpets are manufactured, the fibers will catch particles that normal vacuuming will not remove. This means that the carpet not only holds a large collection of dust and dirt, but also pollen. When these get airborne, they can cause problems, especially for those people who suffer from allergies.
There is a lot of research into why the number of people who have allergies is rapidly rising, especially in those who are under 20. Although not yet proven, it is more than likely to do with the fact that humans now live in a much cleaner environment than they ever have done. This means that as we a growing up from a baby to a teenager, we are no longer coming across the bacteria that can help our immune systems to develop as it should. This is why the medical world is now telling parents to allow their children to eat food that they have dropped on the floor, as the bugs they pick up are beneficial.
However, for those who already have an allergy, it needs to be controlled. If a reaction is triggered by dust or pollen, then having a carpet cleaned by a professional every six months, or so, will make sure that the problem is kept on top of. The cleaner the home environment is for a person who suffers from an allergy, the less frequent reactions will occur.