I have mixed feelings about this. I’d agree that most places that offer yoga aren’t all that great — just today I saw a photo of a local (NYC) yoga class and the teacher was teaching the posture (standing bow) all wrong and everyone in the class had seriously messed up alignment. And some of the corporate chains have been known to push people into teacher trainings. But there’s nothing like having a great teacher guiding you through things. I personally can’t imagine being where I am in my practice without all the teachers I’ve been lucky to have.

Agreed. Great teachers of any subject are so hard to find. I too feel fortunate for the few I've encountered.

There are a lot of quacks around yoga, martial arts and other practices.

That does not mean practicing by yourself is a safe or healthy alternative.

If you can't find good teachers in your city, do something else. But please don't blame a whole discipline for the poor quality of teachers and students.

How do you know what to without an instructor though?

I was not born with an innate knowledge of how to do yoga :)

Instructional videos, or even a book, which is very likely to have illustrations.

Do you learn dancing from books too?

I recommend finding a class that speaks to you. A good teacher will make you get the right focus and postures.

I don't doubt that an instructor who can interact with you will teach more effectively, I was just answering their question. I would say it's possible but very suboptimal to teach yoga or dancing with just words. Illustrations and video improve on that.

People come from all sorts of backgrounds, with different coordination and relationship with body. Classes bring clarity of practice with group. With long experience, words or pictures may be enough. Otherwise the practitioner may be confused.

Many pilates instructors are recovering yoga zealots.