Moodle currently has a "Bootstrapbase" theme, which attempts to map the Bootstrap toolkit to Moodle. That theme is hidden and another theme called "Clean" inherits from it. Initially this was to give people a theme to clone, but possibly it's goals have changed since then.

There appears to be at least one school of thought that believes the default theme (which Clean may become in 2.6) should be familiar to users of the previous default theme, and so a variety of "bugs" in Clean are being fixed by porting things directly from Base/Standard.

There's another trend suggesting that now we have a Bootstrap-based theme we can use our knowledge of Moodle's audience and goals to improve it further and not feel bound to follow the decisions made by upstream Bootstrap and can create our own solutions.

Both of these are entirely sensible points of view, but without at least one theme in Moodle that aims to follow upstream Bootstrap closely, there's no clear indication that Moodle is actually capable of supporting a Bootstrap theme. It's fairly clear that Moodle can support the traditional Moodle way of doing things as it's been hard coded into Moodle for a long time. It's far less clear that it has the flexibility to support other approaches without extreme effort.

External parties who hear that Moodle supports Bootstrap themes will expect to be able to use their existing Bootstrap skills, knowledge and resources to build Moodle themes, possibly to match with other systems that are similarly customized with Bootstrap-variant themes and tools.

I'd quite like to work on improving core Moodle's ability to be themed, and a straightforward Bootstrap theme gives an easy way to measure progress. "Does it look like Bootstrap?" yes/no. The tools and techniques that are created should be immensely helpful to people who want to make a Moodlier Bootstrap theme, or an improved Moodle/Bootstrap theme, but working on all three goals together in a single theme just creates distracting noise.

This work can't be done externally from core Moodle since the required changes are mostly in Moodle's core code.

(I've also just added a similar issue about adding a YUI Pure them to core Moodle. Very similar issues apply and much of the work would be shared)