Is It True That Decaffeinated Coffees Contain Harmful Chemicals?

No, decaffeinated coffee is very safe to drink.  In the early 1980's when decaffeinated coffee first became popular, a chemical called methylene chloride was actually used to extract the caffeine.  This chemical has been linked to cancer, but even then the risks were quite low compared to smoking etc.  
However, it is no longer used as a means of extracting the caffeine.  Instead either liquid carbon dioxide or steam is used to remove the caffeine from coffee beans and neither of these process involves any harmful chemicals.
Moreover, the EEC has very strict rules on how caffeine can be extracted, so there are no risks associated with decaffeinated coffee.  
One of the major benefits of drinking decaffeinated coffee is that since there is no caffeine, the coffee does not act as a diuretic, so it is a useful way of keeping fluid levels topped up: even if it does not have the same 'zing' as caffeinated coffee !