The .row-fluid class creates a clearfix and allows the child elements with a span attached to them to become a percentage width. All fine and dandy, although a little bit more to write than Susy.

Bootstrap Fluid Layout.scss

.main-content {
  @extend %container-fluid;
  
  .content {
    @extend .row-fluid;  
  }
  .sidebar {
    float: left;
    @extend .span3; // Cannot use this type of @extend within a media query
  } //sidebar

  .main-article {
    float: left;
    @extend .span8;   // Cannot use this type of @extend within a media query
    @extend .offset1; // Cannot use this type of @extend within a media query
  } //main-article
} //main-content