The big perf wins:

* Fewer box shadows

* Fewer gradients

* Fewer semitransparent borders (a big performance problem, especially in chrome - I've seen > 10x performance slowdowns when using rgba borders & border-radius: 0 - see Chrome's tracker [1])

* Simpler rules (fewer styles to cascade)

1. https://code.google.com/p/chromium/issues/detail?id=170882

The default look is to make customization easier. So if you download the theme Bootstrap still looks the same.

Edit for typo

- you want people from the companies who are forcing them to use IE, to be able to view the website as intended (and I'm sure they can understand that it doesn't look as good as on the latest version of Firefox)

- you want people in China to have the best experience on your website.

Then I believe you should look at other solutions.