Unfortunately coding for performance tuning leads to code that is hard to understand and maintain.
If I don't write a test, a specification that clearly states what I expect the code to do and see it pass, I can't bet that it will work just by looking at the production code. It would feel like gambling.
"if (userExists === true)".
I used to write desktop applications using Delphi, pyGTK and Windows Forms at the beginning of the past decade. When the web started getting mainstream, it was sad to say goodbye to all the best practices learned all those years. Used to the power of event oriented programming, rich components, data binding, and design patterns in general, developing for the web felt a step back. During the past decade there have been many attemps to mimic the old desktop in the web; portlets, asp webforms, and so on. None of them made developers happy, so the Model-View-Controller (MVC) pattern came to the rescue. However, dealing with the UI is still something developer don't really use to enjoy. DOM manipulation has been a pain in the ass with every browser having different behavior and API. I've been trying to avoid it until now and, to be honest, I am happy to not have invested my time fighting with problems that jQuery handles for me nowdays.
Now, ECMAScript 5 is supported in all major browsers. jQuery and other frameworks make life easier. And there is something that we didn't have in the old desktop programming: the chance to programmatically interact with widgets (UI controls or components). Well there are some frameworks for that in the native desktop, by they came late. This means that we can write automated tests for UI behavior!
So now, we have the chance again to use event driven programming, to really implement patterns like the "observer" and many others. And we can test drive all of them!
We can develop semantic components that add domain knowledge inside UI widgets and make the user experience way better. There is no need for intrusive frameworks that tell us exactly all the layers to be used and how. Everything is ready to craft the code to make it express as much human knowledge as possible.
So we are taking the chance.