I originally had your mindset, but I added a del.icio.us button on my pages for the heck of it. I immdediately saw several posts to del.icio.us to my site and

I originally had your mindset, but I added a del.icio.us button on my pages for the heck of it. I immdediately saw several posts to del.icio.us to my site and surprisingly these were the first posts people had ever made. Even after several weeks people hadn't learned to use the tags yet - every post was system:unfiled (or they didn't find the need to use tagging). As hard as it is to believe, not everyone knows what digg/reddit/del.icio.us are. You're right that existing users would probably already have plugins installed, but the buttons on the page can help new people register.