Decaf coffee contains less of many things, not just caffeine.
In many methods of removing the caffeine, antioxidants are also lowered.
Decaf coffee also has been shown to increase the LDL cholesterol... that's the harmful cholesterol.
Often times, some of the solvents used to reduce the caffeine remain on the beans. They may harm the consumer. Then again, .maybe you enjoy sipping paint thinner. (Methylene chloride)
By the way, too much of just about anything, can be dangerous and bad for a person.