Some great advice has already been given on this subject. I'll try to add some more.
I teach music, it's my profession. I help mostly beginner and intermediate students reach their goals. Frequently, I'll recommend Youtube videos to my students. Sometimes, during a lesson, I'll have them shoot a video of me demonstrating something. Most of them have iPhones or similar recording devices. This can be efficient way to transmit targeted information. Also, in my own experience as a learner, I have used a combination of books, videos, and live or Skype instructors.
Do you need a human teacher? It depends on your learning style and on your goals. It also depends on whether you have access to a good support group who can give you honest feedback. A teacher can give you feedback, demonstrate something for you physically, provide motivation and goals, and provide a way to structure your learning to meet your personal needs. Now, as an individual, and especially with the support of a group, you may be able to get all these things for yourself. If you can't, then a teacher is recommended, provided you have the budget for it.
One other point: will a teacher provide you with information you didn't even know you were missing out on. With a good teacher, moat likely. Should you take lessons just to make sure you're not missing out on anything? I'm not sure this is a strong enough reason. Again, I would look to your goals. Be clear and specific about what you want. Look at a teacher as a possible great way to help you get to where you want to.
I could also write about how to find the right teacher for you, and also how to get the most out of your lessons, but that would take up another couple of threads, at least.
