Pretty wanted to know why the rule is there and not the actual rule itself. OY!
Much is what you said...appearances purposes. Using a whole mess of them really doesn't say much about a post, especially those who are really happy about something and instead of saying that decides to print out tons of emoticons...tacky.
With that said, not knowing if it's possible here, in a few forums, they have placed a program that allows for no more than 3-5 emoticons per post. That may be a solution to that rule if it can be done.