Markup simplicity; easier to style without the extra elements. This article does a good job of explaining the merits of dropping lists on navigation, with a focus on accessibility: http://css-tricks.com/navigation-in-lists-to-be-or-not-to-be
Here's a static implementation of the tabs in Seventy Eight.— http://drupalcode.org/sandbox/ry5n/1932040.git/blob_plain/39e83df:/patte...
<nav role="navigation">
<ul class="tabs tabs--primary">
<li class="tabs__tab is-active">
<a href="#">Edit</a>
</li>
<li class="tabs__tab">
<a href="#">Manage display</a>
</li>
</ul>
</nav>