In my opinion, there is never any need for a colonic. The body does a great job of detoxifying and cleaning out one's intestinal area. Also, there is the possible harm that you mentioned. You are better off thinking about your food intake to insure a proper amount of fruits, vegetables, and fiber, in general.
Hello~
When done right, they are very safe, and a person usually does feel better after because it is cleaning out all the poisons from your system. You don't want to over do them however, they can strip your body of friendly bacteria, moderation is the key word here.
To help them even work better, eat a healthy, no sugar diet, focus on the veggies, fruits and fibre containing foods. Drink plenty of pure water daily as well.