This is why I often say SEMANTICS is about what you MEAN TO SAY, not about tags or placements.
Before HTML5, I GENERALLY structured a page like this:
<div class="motto">most often a client is too confused to actually develop some branding copy and relies instead on sliders (shotgun approach, but ok) so this worked nicely</div>
on occasion i used a cephalothorax (as content and foot being metaphorically the abdomen for some reason) which simply meant I put a wrapper around NAV and BRANDING. remember that there is nothing wrong with having the occasion DIV in HTML5
So it really depend on what the slider is. Think about it this way ... if you were placing a simple motto or mission statement in the same place in the header would you feel such trepidation? So if the slider is just a poor marketing strategy for clients who can decide on an identity and this what they ape putting in place of a mission statement or motto , or list of capabilities.. then it quite reasonable to have the slider in the main header, is it not?
If the slider is really just a 'portfolio', or functional mini pages .. then it most likely just a SECTION .
Hope that helps