1. There is no need to swat a fly with a Buick if you have a fly-swatter handy.
2. If you are greatly over-harsh, then the message will be lost. In their reaction to your harshness, the person will overlook the substance of your message.
3. You can be honest without being mean. Indeed, if you have a reputation for being mean, it will greatly reduce the likelihood that people will pay any attention to what you say at all. It will just be assumed you are venting your spleen without basis in fact.
4. If you want to be considered a good person, I believe that the more likely your message to cause hurt, the more carefully you should deliver it.
5. All this is said with the average person in mind. Sometimes it is necessary to take the gloves off. If the fly-swatter makes no impression, sometimes it's necessary to get out the Buick. (people like oldgrm on Beliefnet require the Buick treatment, most people don't) If the Buick treatment at least teaches some people not to f*@% with you, then it's done it's job.