The CSS objects in a framework like Bootstrap mix elements of style and layout in a way which is very hard to separate. When you go to make changes you find yourself battling the framework.
Take the popular .well abstraction as an example. Suppose you want the layout properties of that object (clearfixing, hidden overflow, etc.) without a coloured background, border and box shadow? With Bootstrap you're stuck, the only option is to right some really bad css, undoing styles already set by the framework.
With Oneweb the framework brings all the layout properties of a .well object, without any of the styling. The UI template adds the style (border, background, etc.) if that's what the design calls for. If not, it's not there and there's nothing to override. The same is true of all Oneweb's CSS objects, the framework brings layout, the UI template brings the design.