The "Add a Bookmark" button that is in the toolbar by default should be a "Bookmark This Page" button:
1. It is the expected that the button will act on the content (the page) and thus bookmark the current page, not any one. This is also the default behavior in popular browsers. (Chrome, Firefox)
2. Making the "Bookmark This Page" button inactive after bookmarking the page gives feedback to the user that the page was successfully bookmarked. Currently there is no feedback that the page has been bookmarked after bookmarking it from the dialog that appears; the dialog closes and nothing happens. The user may try to bookmark the page again, and they will be able to, making it seem like the page was never bookmarked and the feature does not work (bug #767366). If the button was a "Bookmark This Page" button, it would become inactive after adding the bookmark, giving definite feedback that the page was successfully bookmarked.
3. It simplifies the resulting dialog. This would remove the field that allows for bookmarking a different page than the current one in the dialog, making the ui cleaner and making bookmarking look even more quick and painless.
4. It prevents the user from creating a bookmark that will not work. Users will get the correct URL every time because it will be the URL of the current page; the one they want.
I like the reasoning a lot, albeit I have a concern that
a) bookmarklets with javascirpt: URIs would be harder to create since they aren't real pages
b) you can't create bookmarks of redirecting URLs such as gmail.com.
I guess the Add button in the panel should remain the way it works now, so it can allow for those cases while making the common case more agreeable.