The +$2 +1 buy option is only occasionally useful, but it can be a great choice in certain circumstances, especially if there is no other +buy. For example, Spice Merchant/Silver is a reasonable opening if there are Minions available. If the +$2 option happens to take you to $5, I think it's a better to do that than to take the +1 action +2 actions, because you really want Minions and not more Silver. Later, if you've played some Minions and have $8, then the +$2 +1 buy can let you grab a couple Minions instead of one.
I don't think I've ever gained treasure just to trash for the +buy, though. I don't see that being useful outside of constructed kingdoms.
