Actually i think SirClemens gave a pretty fitting example of Good Guy Greg. While it's allowed by the rules to use another players coin tokens, it's certainly a situation that feels unfair in a way. Greg doesn't want to be mean, so he refrains from using them.
I really disagree with that. For me, the strategic interaction of the cards is what makes Dominion fun. To say that you should avoid effective counters out of politeness seems like a poor precedent to me.
I think this is a more appropriate Good Guy Greg:
I think in the case of coin tokens, Good Guy Greg probably thinks that the card should be written so that the tokens are set aside and returned at the end of the turn, and that he's making a friendly protest against the wording of the card. After all, you magically get your Victory cards back if you "trash" them, so why not Coin Tokens? Because cards are tiny, and Alchemy is an older set - that's what it looks like to Greg.
Losing the game on purpose by top-decking the Alchemists is stretching believability, though. There's no similar reason why you "shouldn't" be allowed to not top-deck them.