I can answer that question with one word - FEEDBACK. Don't get me wrong, I use YouTube all the time and encourage my students to do the same. But YouTube is a one-dimensional teacher. YouTube can tell you what or how to do something, but it can never tell you whether you are doing that something right. That's where I come in.
My students ask me all the time, "Hey, Troy, I learned this song off of YouTube, am I doing this right?" I'll have them play the piece for me, and I can give them immediate feedback on how they're doing. That single element of feedback and its application is absolutely crucial for any student at any level of playing ability.
But there is a second, and I think just as critical component, and that is the element of MENTORING. Look, I'm old school. We didn't have the internet to learn from when I first started playing guitar. I learned by wearing out a LOT of records and record needles, trying to master the guitar parts (we didn't have tabs back then either). Thank God for i-Tunes!
I learned much more by MODELING those I was playing with. If I didn't know a guitar chord or riff, they would show me, I'd copy what they were doing, and we would go back and forth until I got it right. That's the way I learned, and this method has been very successful for my students as well. The only difference between then and now is that I can incorporate new technologies like YouTube into this dynamic process.
I also teach my students in their homes. I feel it's more personal that way. We develop really tight-knit relationships with each other, and I'm always available for a quick stop-by to repair a string, or if they're stuck, especially at the beginning, understanding the assigned materials for that week. The relational aspect of working with my students in their homes, and with their parents, has proven to be quite successful - again old-school. And for myself, this relational-triad has been very fulfilling as a teacher, and as a person. I feel like I'm more of a friend coming over for a jam session than I am solely as a guitar teacher.
While the times and technologies have changed, I don't think the importance of that human element of connection has. We want and need to have feedback from those who matter - to know we're doing something right. We want somebody to come alongside and show us, guide us. I'm just putting the human component into a virtual, cyber-age learning environment.
Today, it's all about synthesis and integration. I always encourage my students to do both; YouTube in conjunction with their weekly learning materials. And of course this is where ACCOUNTABILITY comes in. As long as my students are practicing the materials for that week, and can demonstrate their newly acquired techniques, I welcome the "extra" learning my students do through YouTube. I only remind them that their "practice" time is intended solely for their assignments, and YouTube learning is on their time. It works great, and I think the parents appreciate the structure.
There is nothing, I mean nothing more valuable than having that one-on-one connection, especially when playing music. The exchanging of ideas, the back and forth interplay of word AND song. I know of no better way to put it - it's simply magical what can happen. And frankly, YouTube alone just cannot provide that.