I recently had a customer who had had a flood from upstairs, the walls and carpet were damp but not soaking.
John K loaned me a big dehumidifier and his moisture meter, he then informed me to be a bit careful with it as it cost £350!!
For £9.99 I may well invest in one of these and see what it's like, I'd probably feel a bit more comfortable with it.
Agree with Damian, not a massive amount of use in the carpet cleaning industry but I can see a use for it in related areas.