We hear this one a lot. This could not be further from the truth.
This consensus of opinion among carpet retailers (salespeople) is derived from carpet cleaners who consistently leave the carpet too wet. This causes a great amount of soiling to be left behind, which wicks back to the surface after drying finally occurs. This opinion has no factual or proven claims to back it up, it is just that — their opinion, and it is WRONG!
The largest manufacturer of carpet states that hot water extraction(steam cleaning) is their recommended method of carpet cleaning. They also recommend PROFESSIONAL carpet cleaning using this method.