Like all elements in a designer’s tool box, sticky navs have a purpose. Each tool is effective in it’s own way when it is used in the right place. One thing that is necessary to remember is that there is rarely “one” user out there. I find it hard to believe that a majority of users would be using a page up and page down key to scroll a web site. After all, mobile devices and laptops are a huge segment that surf the web, and paging keys do not exist on those devices.

The goal is to make it work for the majority. Sticky navs work because it allows easy access to the rest of the site at any given time, bypassing the hassle of scrolling to the top of a page or hitting a button that auto scrolls you back to the top. That being said, sticky navs should be handled with care and not take up three inches of space. Your complaint may be better pointed at how some designers use sticky navs than the sticky nav as a navigational feature.