The addthis button is different for each post because it will link to the post itself, not the page that the blog happens to be on. The page that blog happens to be on can be bookmarked using the main addthis button in the layout.master file (it will appear if you put your addthis account in Site Settings and if you have the addthis button in your layout.master file). I added an option this morning to allow hiding the add this button in the blog for those who don't want to show it because of using excerpt mode.
I also fixed it so the more link isn't shown if the post is shorter than the excerpt size.
These changes will be in svn trunk by tonight for developers and in the next release for everyone else.
Hope it helps,