It doesn't let you do that though. You have to have cards in discard to get the extra action ands favors so if you are drawing your whole deck each turn without discarding effects you can't get the bonus.
It does let you do that. You don't need to have discarding effects (which notably include discard attacks from your opponent), you can also have your opponent junk you, or you can have mid-turn gains yourself. These effects are not that uncommon, in fact I bet if someone did the math, it would turn out it is more common for at least one of them to be present in any given kingdom. Even in their absence, you can have a deck that sometimes fails to draw the entire deck, which you can always choose to do on purpose if getting to play two terminals is that important, e.g. because they're Bridge Trolls.
Saying it's good if there's no other draw is silly, because that's true for any draw, action, and usually buy, that if its the only option its good. Its still not really a combo, and wont outperform much of any other draw options unless you have discard or some other way to set up the extra effect. It is an expensive smithy without that.
I'm not saying it's good if there's no other draw, I'm saying it's good if there's no
better draw, which excludes a lot of stuff. It's not an expensive Smithy, it's a market price Smithy (see: Swashbuckler, Werewolf, Rabble, Patrol, Journeyman, Catacombs, late game Old Witch, all of which are cards that people buy all the time) whereas Smithy itself is an exceptionally cheap Smithy.