Wouldn't users of those services have the respective plugins installed anyway?

One thing that just came to mind: preparing title, description and tags for bookmarking, but that could be taken from the HTML meta tags as well?

How many users click on those icons?

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.

The plugin is friction: I'd rather a user download the plugin to make other blogs as convenient as mine than download the plugin to get any convenience at all.

And I'm pretty sure those buttons haven't ever cost anyone a digg/reddit/del.icio.us submission.

I rarely click. I do mod on the reddit page though.

Now if only we could mod in the RSS feed.

Bookmark: if I find a post that's interesting and I think I might use it later then I'll bookmark it. digg/reddit: If you find a post thats interesting then you can "digg it" so that others can view it and either agree (digg) or disagree(bury) with you.