Just in answer to original thread topic post:
I train with a friend without formal instruction for the most part. I've taken about 3 months of formal Muay Thai, but he has only been to 1 seminar. We have a ton of MT videos, though, basically everything you can DL from the net.
Sparring tends to be overemphasized in our training, I think, because my friend is a little lazy techniquewise; he doesn't believe things like head position when kicking matter until he has it proven to him by being hit when he kicks, etc.
Actually, for me, this is a real bummer.
Basically, the problem is that he hates to drill, and that he is too results oriented. That means that if he isn't getting through against me, sparring, he keeps turning up the speed until he does, and that leads to injuries for us both.
If we had a formal instructor, we could avoid some of those tendencies because the instructor would be able to say, "No, that's good," even if he is not getting through, for example. I try to tell him this, but because I'm not an instructor he by and large ignores me. And this is an intelligent person, and a good friend of mine.
I think there are alot of injury related risks my friend & I have to deal with just from not having an instructor to lead the drills and break them down in their details, particularly in countering and in the finer details of each technique.
I think in fact one of the reasons I am able to use drilling more effectively than my friend - e.g. my countering is much farther along than his - is that I have more instruction than him, in a class setting (i.e. when I am drilling I know that the technique itself and the muscle memory is what I need, that this is where effectiveness will come from - I'm not sure my friend does, because I think he is looking past the drills themselves when we work them. I can't change this as merely his friend, not a qualified instructor).