But the problem is that human communication is heavily reliant on body language and facial expressions. So, even with the most expressive of writing, it's often still almost impossible to judge someone's mood or intentions, when they're telling you something.
Using smileys acts as an effective medium to indicate someone of their mood, or intentions, with the mere flick of a few keys, whilst avoiding being creepy by constantly telling people how you feel about things, wich is why I believe that smiley's are incredibly usefull in written / typed conversations. And, since human emotions are very diverse, and complex things, I believe that there should be no limit to the complexity of a smiley, as long as it is readily available, and effectively get's the message across.
For example, if you're reading this, you might think that I feel slightly pissed off, due to my extensive use of expensive words. But, all I have to do is a place a :I somewhere in my message, and it immediately tells the reader that I actually feel somewhat insecure about my statement.
:I