Can you add a Twitter Bootstrap tag?
Bootstrap framework is no longer associated with Twitter so tightly as it used to be, it's no longer called "Twitter Bootstrap" in their own documentation, so including twitter in tag seems pointless. On SO it is used only for consistency with twitter-bootstrap-2 tag.
For the need of the tag itself, I do not think it's needed:
Questions that are really Bootstrap specific will be PHP / JS / CSS question that should rather be asked on Stack Overflow, with tag that already exists
Questions vitally about Drupal part of the job will hardly ever (if at all) be specific to Bootstrap Framework, so theming should be enough for them.
I can't imagine Drupal Answers user willing to answer only Bootstrap question, or specifically ignore them. Reason: Point 2. So usefulness of this tag for filtering is dubious.
I could potentially see the usefulness of a tag for
There is a base theme, and I can some potential questions about getting the proper classes into the right place, and possible some other things related to the base theme (but not general Bootstrap questions).
However, I would want to see enough questions to warrant the new tag.
I would also want to really think about what to call the tag. I don't think bootstrap is a good tag, as it is too close to bootstrapping, which has a decent number of questions and IMHO is a more important tag for understanding Drupal.