Site/page navigation takes up too many lines, especially on single-column desktop mode.
From Skin talk:Timeless:
There are simply to many layers at the top in the half-compact view. You have site (with search and personal), site navigation, page, and page navigation. Four levels? Add that some pages at Wikipedia has their own levels. This is simply way to much!
Add site navigation as icons at the site level, and likewise add page navigation as icons at the page level. That would shrink the top to two levels, and make it more manageable.
In the very-compact view there are icons for this, but the line for page navigation is still there. That leaves three levels, which is one too many.