I am also using bootstrap. Can’t say if it is the best cause I haven’t tried all but will probably try Foundation soon. But generally I am pretty happy with B haven’t had major hurdles. The only drawback that bootstrap has is it doesn’t have speech to text so I don’t even have to type my layout 
Althoug I like bootstrap and use it in almost any project I dont think it has to be part of the framework. May be released under some official extension but not part of the core fw. This just bounds the fw too much to something that may not be so good in the near future.
The interesting part is how to get themes from extensions and activate them in your application.
How are you achieving this now?
At the moment (Phundament, Yii 1.x) I copy the theme files from vendor into the project and use my multi-theme extension to switch between them.
For Yii 2 we can use aliases for the theme switch I think…
Thanks for your information. 
Bootstrap 3.0 final has been released http://getbootstrap.com
Excellent release, I won’t miss Foundation 4 features.
I’ve updated Yii2 to use release version.