An acronym is a slang or shortcut to commonly used phrases, while the emoticons are symbols to express an emotion. Emoticons help you to express the tone of text that could be ambiguous. Sometime when something is meant to be light hearted or sarcastic can be interpreted as mean-spirited. This is where the symbols can be used to relay your intentions to the reader. Without that smily face the tone of the reply could be taken as a negative reply.
Emoticons are easy and fun to use.
I find that I can express myself better when writing E-mail rather than a written letter, because I have the use of these tools. Although be careful not to rely on them alone, they are not meant to be a substitute for a well composed written message. Replying to a message also has it's own style. E-mail is asynchronous communication meaning it can be moments, hours or days before somebody responds to you message. As time passes you may forget your question or comments.