Robert Jasiek:
Then he will understand that assessment of thickness and territory are not difficult but assessement of complex fights can be difficult when what starts as a joseki creates a large middle game fight.
So you're also agreeing that the middle game is more important than the joseki that got you there. In other words, how you navigate the outcome of your opening will bring you the win more than the opening steps.
But also, the assessment of thickness and territory ARE difficult for joseki. Thickness changes based on surroundings. That's not an easy thing to really grasp.
MagicMagor
Also i don't like the a joseki mistake makes no difference in an amateur game argument, because it sounds like the opening doesn't matter at all in amateur games. But what strength is meant here by amateur? For a 20k or DDK game it may indeed be true, but what about SDK or dan-players? If i don't know any joseki and make a 10-point mistake in each corner of it, i'm 40 points behind going into the middle game and my opponent is leading by 40 points and this doesn't affect the outcome of the game at all? Actually i find that quite insulting.
Amateur is just that, amateur. These problems apply to even dan players. If you find me just saying that insulting be prepared to be insulted some more, haha.
Dan players make the same fundamental mistakes that kyu players do, just in different ways. This includes joseki. Even dan players will say "i started out so good, but somehow i lost the game because of some middle game fighting". It might be less obvious, but it's still there.
But let's take your point, where you've got a small loss in every corner for the opening. If your opponent is around the same rank as you, there is no guarantee that even he knows that he's a little ahead because of opening choice. And even if he does, it doesn't mean that the middle game fighting will go his way to where he can maintain that lead. Let's also stress the fact that a "10 point mistake" doesn't exactly always mean 10 tangible points. It could be applied in a way that is realized in thickness or aji. Don't take that number and think that it actually means he's ahead by that much. It's a little bit more vague than that.
Magic Magor
If I make a big joseki-mistake and are behind during the middle game but then miss the chance where my opponent made a mistake allowing me to come back and lose the game. Did I lose because I made the later mistake or did I lose because I made a joseki mistake? On which mistake should I focus my study?
Both are valid issues. The middle game problem is a bigger issue that can be applied in more areas more times in future games than simply a "don't do this" problem in a joseki. Thus, middle game lessons are more important than a joseki lesson here.
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Here's another way of looking at it. Yilun Yang once talked about this as well. He showed a opening position and argued against this one move that the player did.
The player responded with: "Well Go Seigen plays this and says it's a good move.
Yilun Yang said: "Well, if it's Go Seigen i won't argue with him, but do YOU understand this move? If you don't understand this move and what it's doing its not a good move. Just play the move that does something you can understand"
Joseki is the same thing.
If you don't know what you're getting out of the joseki you're playing, you're doing yourself and your game a great disservice. If you play within the bounds of what you comfortably can understand, it is better than playing the "correct" move that might lead to a variation you can't handle. (AKA, learning joseki really isn't that necessary)
Let's also look at it this way. For joseki to get "complicated" it requires both players to get into that complication. If you play simple responses the difficult variations will never occur and you'll never be in danger of losing because of "joseki".
If you show me a game where you were at a disadvantage because an opening joseki, i will probably ask you "why did you choose this joseki, what were you thinking going into it, and what were you trying to do while play it". Chances are you were playing something beyond your comprehension.
There are plenty of players in those countries who learned by the seat of their pants and got quite strong. Not so many in the West, I think, but that is changing, as we get a larger pool of strong amateurs to play against, as well as strong opponents in the East. 
isn't played at a and how to punish it, is part of knowing this joseki.