I have played Insane opponent only once or twice and it was before the AI got improved with the patch. On one side it is strange that you cannot beat insane opponent all the time if you can beat it once. It is not like the opponent will adapt to your strategy like a human would do. Maybe you have not cleared out your strategy and are changing things all the time if you sometimes win and sometimes lose.
It is certainly not true that everyone playing multiplayer can dominate the computer on all levels. Higher level players certainly can but I have hit many players that I am confident have not even played the tutorial and the first thing they did was hit the multiplayer button.
I already mentioned one reason why you may want to play multiplayer before achieving 100% win rate. It is that real world opponents adapt. After playing several games with one player you may want to tweak your strategy to his style. You will get into mind-games. There is no way that playing against AI will prepare you for this. You need to be able to read the small signs and infer what the opponent is doing.
Finally specifically the Insane AI cheats. It has more income than regular players. As such it is not a good way to practice the overall strategy (it may be good way to practice unit positioning and tactics but not strategy). This is why I never play the Insane AI. I sometimes test little things via weaker AI.
If you are interested in more information you may want to check my Armed articles:
ARMED! Strategy Guide Part 0
ARMED! Strategy Guide Part 1
Sadly the current state of the game is a russian roulette of desyncs. I rarely play the ladder these days because as it turns out the best strategy to stay on the top of the ladder is not to play (if you are under the middle of the ladder it is good to play) but we know that a fix is coming soon. I certainly recommend playing competitive games.
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