I think main reasons why (some) people dislike mirror go are:
Point 1 is a generally misconception as pointed out above, although it may be true at DDK level or a bit higher.
Point 2 may or may not be true, and is probably false if the opponent is about dan level.
I think two sources of misunderstanding are:
Unless it’s all just a moat and bailey tactic. But I wouldn’t assume that, just because it’s possible.
Please correct me, if I’m wrong.
Most DDKs know about tengen as a breaker. However, they may also dislike the time advantage gained by the opponent.
Point 2 merely rephrases what I said, and before me Jeth, about it not being earnest play at the weaker levels. Of course it’s true in the vast majority of cases. Higher levels were already ruled out, so that twist is irrelevant.
The psychological trick may also play a part. It’s like being jumped from behind and having a knife held to your throat. People tend to resent that.
I didn’t notice that he (or other people than you) were talking about weaker levels or ruled out higher levels. It would be clearer for the discussion if people who make statements about mirror go say if they are made “in full generality” or just for particular levels.
That sounds really exaggerated. But I agree with the psychological part. If you are mentally unprepared then you may feel helpless against mirror go, and this can make you play more badly than usual.
Do you have evidence? I don’t see a valid reason to believe that. A DDK can also play mirror go in a serious and honest way, it’s just that their judgement about when / how to break the mirror is on a corresponding level. It seems seriously unfair to just assume that they are not engaging with the game, or that they are not playing the game honestly.
So this is prejudice and I see no argument to justify it.
No we did not say that, you said that … And it’s seriously a terrible thought.
“Oh there’s a person who I know nothing about, so it’s ok to make any assumption I want.”
If it helps, the pro/high dan levels were mentioned early on at the urge of @Groin who pointed out the fact that it is a valid strategy “in titles tournaments”. I pointed out that at that “in titles tournaments” only the victory matters and a pro is obligated to try to win regardless of the opponent’s strategy, since that is their job. Therefore we are mostly discussing about amateur level (which is where we are), since unlike a title tournament, in an amateur’s case there can be many more reasons on why someone is learning Go and sitting down to play a game in their spare time. Post 40
I’d like to add that I hardly think that our opinions hold any validity on levels of play that we have not or, to be more precise, cannot reach 
If you want to quibble, fine:
We were discussing that, I made that point, noone managed to refute it.
What’s “terrible” about that? 
a) We all do it all the time in real life. Spare me the preaching.
b) No assumption is made on the personal character of the player. Only on their goals on the board. And we do that all the time as well, it is part of the game trying to decipher what the opponent wants on the board.
Good! You are half-way there in understanding my main point:
Outsourcing your moves to the opponent is saying that “I am not playing in the first moves”.
The opponent standing up and resigning is saying “well, I am not playing the rest of them either”.
Now all you have to do is apply the same logic that you just wrote, on the mirror Go and you will see the lack of engagement in both cases.
One step at a time 
I can understand him just fine
I think Engagement was fine; you specified in what sense you were using it, if there was any doubt
I don’t have to apply logic in a way that I’m convinced is wrong.
As you wish.
This is known as putting the cart before the horse.
With this you are admitting that “logic” is only good as long as it props your pre-existing convictions. Once/If the same logic could challenge your world-view, then you willfully discard it, instead of reconsidering.
Far be it from me to judge that, but hey, if you had told that from the start, a lot of posts and time would have been saved
We can always argue semantic but that’s clearly not how Jeth (or Conrad) are using the notion of engagement.
Logic pedantism here, but that’s not what Martin means here (I think).
You’re asking him to apply your premises to his argument, not to apply your logic his argument.
That is, the premise that mirroring a move is outsourcing it to the opponent, and thus lack of engagement cannot be interchanged with the premise of not playing a move, which is lack of engagement.
There’s not much logic there to begin with, it’s more a matter of opinions.
I’ll break up the random nitpicking of things, to mention my main experiences of mirror Go:
Yep that’s something interesting to explore and I would even recommend it for the weakest players because it’s easy to “punish” the opponent who don’t want to think on his moves.
At higher level you will face the possibility of your opponent breaking the mirror because let say for example that 2 contact moves on the tengen may be less valuable as a move in some corner.
Sorry but that’s not taking in accounts mirroring games with tengen already occupied.
So what? It doesn’t need to account for all the mirror games in the universe.
There are only two kind of mirroring and excluding one seems to me quite a shortcut. That’s all what I am saying.
I think I finally understand why some people don’t want to play mirror go.
In light of shinuito’s post, mirroring the opponent’s moves may allow to win against much stronger players. It rarely works, almost never against dan players and perhaps only once against high kyu players but it sometimes does, so using this trick, albeit legal, is “immoral”, unless you are in one of the following situations:
In all other situations, this explains why Jeth Orensin wants to resign early. If his opponent desperately wanted to win, then he gets his rating points and everybody’s happy. If he did not, then this sends the message that he should have asked permission first, to avoid any ambiguity about his intentions.
I hope I didn’t misunderstand again… If I did then I give up.
I strongly disagree. I love when a DDK has ideas to put on the board even when I can refute them easely.
You think you can use this mirroring to beat me? Look, it’s not going to work at all. And I’m not a mirror master so you know if we can enjoy some higher levels of mirroring that will be perfect. Although yes, I am bit far too optimistic. Anyway I hope this game gave you a bit of perspective on how deep can be mirroring and in a more general way the game of go itself.
There is no trolling, there is bias to see troll in everything.
A bunch of the discussion is about if it’s “immoral” or not (and why or why not)
There’s probably different reasons to dislike mirror go so I don’t want to speak for everybody, but from my perspective the opponent’s mind is a secondary consideration.
The key point is that I find Mirror Go has the unique trait of feeling almost like a different game than regular Go; a game I very much do not enjoy and do not find fun to play, in part due to the lack of “engagement” it generates from the perspective of the non-mirroring player (to re-use this popular term).
For further clarity: