That being said, I started using it extensively after being a big fan of Instapaper. The browser integration (especially the easy tagging) has been really helpful in keeping my tab count down.
The only drawback, for me, was the added bulk of the Bookmarks bar. I like to slim it down, so having the single button has been great. (Also, being able to add the tags, but maybe the bookmarklet is capable of that?)
When you click the bookmarklet a little dropdown banner shows and you can add tags from there.
https://addons.mozilla.org/en-US/firefox/addon/custom-button... ... ah, and now I see it's unmaintained and recent version isn't working for some, carry on, never mind (works for me though).
They don't want it there at all, in any way. They don't want it using any of their computing resources, no matter how minute this usage might be.
They tend to see it as something that should be available as an add-on that's installed on-demand by those who actually want the functionality.