First of all, let me dispel a myth right now, Decaf coffee has LESS caffeine, not NO caffeine. Decaffinated coffee, for instance, must have most caffeine removed, but it can still hold a lot of caffeine if you drink it in bulk. So while drinking decaf tea or coffee might be a wonderful choice for someone suffering from Insomnia or Acid Reflux, it is not a cure-all or a wellness drink.
Many people have been worried that decaf products are bad for you, simply due to one of the methods in which caffeine is extracted from the beans. Of the four methods, using methylene chloride, ethyl acetate, carbon dioxide or water, the methylene chloride method has been mostly criticized because inhaling this chemical as a vapor has been known to cause cancer in rats if inhaled. However, drinking this chemical has yet to find a single harmful effect, and the amount of residue in the bean after the decaffeination process is so negligible that any fears are not rooted in reality.
Since we know that substance toxicity is cause by concentration, not mere exposure, it is important to point out that “exposure of children to methylene chloride in drinking water should not exceed 10 milligrams/liter (mg/L) for 1 day or 2 mg/L for 10 days” which is the same for decaf coffee, which must not exceed 10 Parts Per Million, or in other words 9.8 milligrams/liter. So unless you drink a more than a liter of decaf coffee a day (4 cups a day), you should be OK.
Coffee is known to have health benefits, and a lot of them are connected to the anti-oxidants found in coffee. While decaf coffee has less of these, it is still OK to drink. I will not say that decaf is a health drink, but like I said before, people suffering from insomnia or acid reflux would do well to steer clear from caffeine, and decaf is a way for them to enjoy the taste coffee or tea without having to worry about caffeine messing with them.
But in conclusion, no, the decaf process is not a poisonous process that causes cancer, like some health bloggers may suggest.