Such transcriptions has become part of the written informal language. If we have no problems using informal spoken language in informal situations, why shouldn't we use informal language while
writing in informal settings?
I see various written representations of laughter on the internet pretty often, and sometimes I may become quite confused as to what the word means. I remember seeing "heh"... I had no idea what it meant and looked it up in my old English-Russian dictionary, and it said that "heh" expresses sadness or something like that.
And I haven't known its true meaning until this day when Skrej thankfully explained it to me. Thanks, Skrej. With your help the mystery was solved.