This is my conclusion - see previous post.
Multiple exclamation points have their place - in amongst hearts and flowers and smileys and emoticons and xxxx and x0x0x0x0 (to the uninitiated that's hugs and kisses) - in other words 'nice, girly gossip'.
OK when I say 'in amongst', I don't know many who could stand them all at once - but I hope you get my drift. I hesitate to classify (ergo validate) them as a genre.
When used on a fairly serious or businesslike forum, however, they look silly or sarcastic. The Gosh! Honestly! No really! effect that causes a warm giggle in chatty posts transmutes to a demand for attention, a suggestion of outrage or at least foot stomping and/or arm-flailing.
It also matters if they are used at the end of what is obviously a complaint or personal attack. That's exactly what happened before that idiot explained himself by saying that multiple '!'s meant humour.
What he said, in effect was "I turn my back on this thread and look, instead of serving my needs it's been hijacked by X's problem!!! And blah blah rhubarb as well!!!"
That sucks. That's too low to be called sarcasm - he was being an out-and-out drama queen.
So, to answer my own question: do you see the use of multiple exclamations as 1,2,3, or 4 - the answer is Yes!!!!!! All of them, depending.