All ebooks I read must have a working TOC\Navigation.
I can't tell you how many times I hit the back button and lost my place.
Also on Kindles a TOC is a must to be able to navigate to the Previous/Next chapter by hitting the left or right arrow key.
One of the first things I check before purchasing a self published books is that it has a TOC if not I'll have to add it through Calibre. Sometimes it's easy by adding the right xPath and sometimes I have to extract the book then open it up in Word and mark the chapters with the right tags. So I am willing to pay more for a book with proper Chapter headers and a working TOC.
The Kindle Fire does not have navigation like other Kindles so a TOC is a must have to be able to navigate a ebook.