I disagree with both of them on a lot more than just programming practices.
They saved quite a few bytes by cutting back on privacy.
In the normal case you're [almost] absolutely right: performance and scalability are negligible items, provided that you're not doing something that performs in exponential time or something.
If you're looking to keep your clients (or your job) when your sites get slashdotted or dugg, you'd better have efficiency and scalability in mind. And no matter what, you ought to have "best practice" in mind. If you've been told otherwise, you've been misinformed.
I doubt anyone could maintain their employment at a long-standing highly trafficked site without attention to best practice. Amazon, Facebook, Yahoo, Google, LinkedIn, etc. ... they don't hire PhD's to let their subordinates "do what feels right." They could hire kindergarteners for that.
But again. For most websites, you can certainly be a little careless and it won't cost you a thing. (other than the ability to more effortlessly produce the "best" solution.)