It's interesting to watch how they work and the kinds of outputs they produce. Quite a lot of the stuff works... mostly...
It's entirely untestable, not repeatable, undocumented and just stuck together with flaky glue code.
Sure, if you aiming to "make it work" you can do that. If you're aiming to do well... I suggest learning proper technique.
Code that's testable, repeatable, well-documented and elegant doesn't guarantee that an application will actually meet anyone's needs in a commercially sufficient manner.
The WordPress code base is frequently criticized, and the Facebook index.php code from 2007[1] will probably make some people cringe, but these are just two of many examples demonstrating that if you want to "do well", building applications that work and meet the needs of their users is initially a lot more important than "proper technique."
Testing and documentation make a difference in the long term. Unless your business model is "build a shiny tower and sell it before it crumbles", at some point, you'll have to fix the mess you made in the early days of the application.
I have talked with some developers about my observations and almost all of them had a similar anecdote to tell. Some of them included CS grads with a similar "good enough" mindset.
These anectodes are in no way a representable study, but for me, it put the radical professionalism movement of Uncle Bob in a whole new context.
I have a Software Engineering degree (certified Engineer), so I already have a superiority complex anyway ;)
Proper technique and best practices are definitely important. They can, however, be a part of the self-learning process, just like learning how to write the program itself.