for me, it's obvious -- ease of use.
you can be reading a post that is really good.. maybe not the 1st post in the thread.. you like it, you 'digg it'.
no need to go to Top and book mark.. just digg it right there.
that's how i have my social stuff done. (i use scriptac, but it's the same idea)...
i css the 'Top' link to the right and have the little bookmark bar on the left for each post in the thread.
workie nicie