Its a wise thing to do. I use it to simulate, how I would do in a face to face communication. So a smiley, where you would actually smile, to soften the blow, or to add humility when pointing somebody's wrong could be useful. Of course its use has to be limited, else it will look like a typical WhatsApp message emoticon train. :)

There are lots of small things, which if more people do, would lead to friendlier communications. E.g. replying with a greeting, to a mail which has greeting.