It seems like I receive a request for a fixed header fairly regularly more than I should. I don’t really understand why you would want to fix the header of your site. So I figured I would layout some of my opinions on the matter I may be wrong but this is just my point of view. Full disclosure this sites header is sticky on Firefox purely to use the sticky CSS property since I use my site to try things out on I believe this is okay. So yes there is an exception to every rule and opinion.
I understand why you would want to fix your sites header when you have a single page and you are using it to navigate on the same page. I do think this would be the only valid use case I could see. A fixed sidebar navigation would also be valid in this case on larger screens. However this does not happen often so why are there a lot of fixed headers.
I see this trend more on mobile sites than on desktop which begs the question why is your logo so important that it is taking up 10% of my screen space? I say your logo when I landed on the page and if you have a back to top arrow instead I can use that to get to your navigation. The weirdest place I have seen this is on news sites. So you are telling me half way through your article I should go to another page. I have a better idea how about I go to another site.
I do somewhat agree with the sites that use a fixed header when you scroll up the page. Since if you are headed up the page there is a better chance that you are trying to get to the navigation. However, this is not a license to fix the header when you scroll up and down no matter what you do it just seems to misbehave. I think sometimes we forget that the two most important parts of the site are not the designers, developers, and owners. However, they are the content and the users that make the site great. So why would we cover our content and frustrate our users just because we think something looks cool.
I am not sure where this trend comes from maybe trying to make it more like a mobile application. Well in that situation build a mobile application where the navigation is primarily constrained to views in your application. Sure they can link out to the web using web views and links to the devices browser. Most of the navigation in an application is from many smaller views instead of larger form content. So in this situation, a fixed navigation makes more sense since you are spending more time navigating around the application.
This is a trend that I would like to see less of in the next year I doubt I will. If not, I hope to see more reasoning behind why we would cover up part of our content and frustrate our users. Decisions at the cost of usability even a small amount are the wrong decisions to make. Just remember there is an exception to every rule.