I just don't understand the argument that we should all just use HandlerFuncs for everything. Python and Ruby have stdlib HTTP facilities that are pretty good. They aren't nearly as nice as net/http, but you could totally build websites with them. People still have written lots of web frameworks, because it makes their lives easier. It lets them express programs in a simpler, higher-level way. I feel like this is even more valid now in 2013 since nearly anyone who's building websites has done so with one of those high-level frameworks, and they expect a similar experience.
Lots of people have abandoned Rails for Sinatra, or Django for Flask. That's totally cool, and there are good reasons to do so. There are also very good and obvious reasons to use something that further abstractions common patterns if the software you are building conforms reasonably well to those patterns.