Well, I did go for taxman not for the copper upgrading, but more for gold into platinium. Since it's a game with bunch of golds, I thought taxman would often hurt my opponent, and discarding a gold at the start hand can really hurts more than losing the gold in a hand with bunch of cards, because usually for the first action of your turn you want to Apprentice a gold or something like that. So, maybe not a taxman for the opening, but afterwards ?
I still think I have played better than my opponent and 1st player advantage was a big part of the game
You both played terribly. Not in a "you're bad at Dominion" sense, but rather in a "you don't grasp the power of App-MS" sense.
You played a sort of App-MS Big Money strategy, filling your decks with treasure and then buying VP over several turns. Taxman is plausible in such a strategy, and you're defending it in that context. But the context itself is invalid, because with or without Taxman, App-MS played as BM will get absolutely crushed by App-MS played as a megaturn strategy.
Look more closely at SCSN's game, and then try a few practice games replicating the plan. It'll quickly become obvious that it's far stronger than the plan you played, and also that Taxman and Platinum have no place in it.
Edit: Here's an
old thread with further elaboration of the strategy and lots of sample games (including the one time I've seen this strategy beaten in a non-mirror, a tournament game I lost to Davio).