The attack function of Margrave is exactly the same as if you played a Council Room, then Militia. A stack of Margraves is the same as a stack of Council Rooms, then a Militia at the end, except that it's marginally better because your opponent has to draw and discard each time and may not be able to tailor the best three-card hand.
So, sure, the attack function of Margrave doesn't measure up to, say, Torturer. But it's plenty strong enough to be the backbone of a +Actions/+Cards engine- it attacks, draws, *and* gives +Buy all at once, something no other card does. It may not be quite as powerful a card-drawer as the Council Room/Militia duo, but the fact that it only takes one card to do what would normally require two more than makes up for it.