Minion‘s attack is a bit of a gamble. Sometimes it hurts, but other times it actually improves an opponent’s hand. But it is quite powerful when you know your opponent has good cards in his hand, and this commonly occurs when your opponent is investing heavily in Treasuries. The effect is doubly devastating: not only are you getting rid of one of his primary sources of income, but by forcing him to discard them, you prevent your opponent from chaining them together again for a long time.
Minion also counters Alchemists, and for the same reasons as outlined above, can completely wreck an Alchemist’s ability to get through his deck. To a lesser extent, Minion can also nullify the top-decking power of Royal Seal/Watchtower/Herbalist (assuming they top-decked something useful, you can then force your opponent to discard it immediately).
On the same principle, Masquerade can work in place of Minion, albeit via a different mechanism. If your opponent’s hand is consistently filled with Treasuries/Alchemists, you can often force him to pass you a good card.