I use the add button in a few places, it doesn't seem to add a lot of value, and I doubt it improves your chances with any of the search engines.
What it does do (or can do) is allows the visitor to click it and then choose which of their social network sites they are going to submit it too. At the bottom of each post here are links to 5 of the common social network/bookmark sites, Digg, Del, Technorati, furl, stumbleupon, though these are five seperate links.
With the 'add-all' bookmark you click on the one button and then have a choice of lots more social network/bookmark sites. This thereby increases the chance that someone will bookmark the page on their preferred network.
It depends what you mean by 'register', if you are the first to submit the article most will require you to be a member and also to write a little blurb, some may ask for suggested tags, topics. Each one is different and does require you to be a member to be able to submit to them, though one other thing in common they have is none of them like you to only submit content from one site only
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