I'm going to tell you why we shouldn't use the word retarded. Retarded is defined as: less advanced in mental, physical, or social development than is usual for one's age.
If you were born with a condition that made it so, you develop slower than others. Would you want people pointing it out all the time and using a simple word to speak down to you? No.
You may say that using the word doesn't offend these people because they don't know what you are saying anyway. Wrong. Just because someone might be a little slower than you doesn't mean that they don't understand and hear every word you are saying. Most of the time, they do, loud and clear. For example, I know a child with very low functioning autism. He is nonverbal and certainly has some development that is slower than others. What he doesn't have though, is a small brain. Recently, nonverbal children have been given the opportunity to learn and speak with a computer. That way they can form words and sentences that may be kind of hard for them to get out through speech. When this nonverbal child was given this technology, he formed a sentence right away. As true as anything can be, he typed out "I know more than you think." His friends and family had been speaking down to him and helping him with everyday tasks that if he could have spoken so, he could have done on his own or with little help. Case in point though, he understood and heard them. That's one reason why we shouldn't use the word retarded.
This word is not a joke, it's not funny and even though you may think it is or not mean anything by it, chances are somebody around you has negative feelings toward it. Imagine if you have a child, niece, nephew or even friend one day with special needs and you hear an individual around you use the word "retarded" in a degrading, mean way. It's going to sting and bother you and you will kick yourself for using it the same way before life handed you the real thing.
Please take the initiative and pledge to help stop the use of this word. Spread the awareness that this word is offensive and should not be used unless in medical terms. If not for yourself or someone close to you, do it for those who have special needs and hear somebody call them "retarded." Think about how it would feel if there was nothing you could do about it but you are being punished verbally anyways. We are all human, we all deserved to be loved and treated the same as anybody else no matter how we develop in life. So please, pledge to stop. Let's kick the r-word out of our vocabulary.