TirlbyI had typed up a pretty long response before I accidentally left the page, so I'll just try to get to the gist of it.
Slin, I agree that mentoring can be unrewarding. However, the newbie mixes allow players to not only get feedback from coaches, but make connections with them and other newbies. They are meant to introduce players to 6s, and provide a place for more experienced people to play. I've made friends that I enjoy playing with from newbie mixes, and I've gotten to know some of the coaches too. It's basically an open pug group and I think it's important to keep it so newer players can have a place to meet new people and play pugs. Since the community is so small connections are quite important. Honestly the newbie mixes are more about hanging out and playing with other people; the learning aspect is just an added bonus. People who enjoy them will stick around and come back. So through the mixes people will get the chance to try out 6v6 if they're new, and continue playing it if they enjoy it. Basically, newbie mixes are better for introducing the game to newer players than tf2center, and are a way for people to play pugs that aren't as high skill as tf2mix.
+1 to all of that. Newbie mixes are great not because you teach newer players something they could not have learned by watching youtube videos, they're nice because you have someone you can actually communicate with and it's a much more personal experience. I know I got into newbie mixes after I played one where you coached me and it was pretty cool to see a name I recognized from eXtv casts teaching me how to play TF2.