Revealing HT to an opposing Soothsayer leaves you with a 7 card hand. That's pretty good for HT if you can afford the extra terminal. On alt-vp card boards, Horse Traders is often a terminal of choice.
Now, why you'd get Horse Traders purely for the reaction is beyond me, as all it effectively does is replace itself as a reaction for non discard-to-3 attacks, and makes the discard-to-3 attacks give you a +1 card at the end. Little better than Moat. It's equivalent to getting Great Hall as a defense to non-discard attacks unless you want a big handsize regardless of card quality.
I just realized that Soothsayer must be really tough to ignore in 3-4 player games. Having a 7-8 card hand on turn 6 doesn't seem like it offsets the extra curses and the lack of Gold relative to your opponents in a 3-4 player game.