Bootstrap CSS adds margin-bottom to some components and margin-top to others. Then if, for example, you start a new row with a h1 (which has margin-top) it will be "natively" pushed away from the previous component creating a good sense of component separation right out-of-the-box. In contrast if you start the row with something like a div.carousel (which has NO margin-top) it'll stick to the previous component without any spacing in between, breaking the rhythm reached so far.

Also double margins create collapsing margins very often. Thus we have to create unnecessary "spacer" divs and custom CSS classes either to add margins or to keep collapsing margins under control.

Harry Roberts has made a great work explaining why this is important: http://csswizardry.com/2012/06/single-direction-margin-declarations/.

Now can vertical rhythm be re-considered to eliminate margin-top in favor of margin-bottom only?