They mostly add to spam activity, so I always keep my smilies to a minimum, and make sure most of them have some sort of theme. Basically, not a bunch of random smilies found on Google.
For my next project, I'm thinking of two things:
1) Completely get rid of smilies, since on many of my forums they are not used anyway. Many people seem to just use text smilies like: =)
2) Create custom smilies to match the theme of the website.
I agree with Tecca. I can't stand when i goto an active forum, just to find there's a succession of smiley spammed posts. That's it, just a smiley,and no text.
I keep my smiley's minimal, and generic. With one smiley per emotion, i guess. There's no need for 8 different smileys with the same meaning ... like happy.
Some forums they work better for. But i don't think adding smiley's is what contributed to any boost in your forum, unless they're spamming smilies.
And, I'm not fond when i see the "Welcome" smiley when someone replies to my intro topic. Shows more laziness than actually saying "welcome" and posting it.