I think the original reason for requiring 10 character posts is to limit "posts" where "nothing" is said. It looks harmless but is not; look at a post box, all the information that is there, your name, avatar, poster information, signature, color, text, etc. choices. Those require "space" and space costs money and you want me to scan through three+ inches of all that for a smilie?

We have the "Thanks" button if we like or agree with something someone else besides the OP has said, thanking them for posting what they did, I would think 10 characters would not be too much to ask to post to an OP on a post they've made that is longer than that? If you have nothing of your own to say; don't say it!