Do you dust under 2 if you think you won't use them? I generally dust Golds out first (better cards sooner > shiny cards now), and then just auto-dust to 2 owned.
If you keep copies beyond 2 of a card and that card gets nerfed, you get a dust bonanza when you dust the extra copies.
Unstable portal doesn't seem nerfworthy.. or playable.. to me.
I haven't played the card yet either, but from testimony it's a lot better than it looks.
With some small chance, you get random 0-1 cost that wastes a card
With some middling chance, you get a random 2-3 cost that is at least okay.
With some middling chance, you get a random 4+ cost that's alright.
With some small chance, you get a card that breaks the game.
Basically, on average you expect to get some 2-3 cost that's just alright. If you pull a 4+ cost, it's very likely to take-over the game. Essentially, you think of the card as a delayed Innervate. If you pull, say, Spectral Knight/Spiteful Smith, you can drop it for 2 on turn 3. If you pull a 4 cost, you can drop that 4 cost + a 2 drop on turn 3, so even something like a 2/5 body can be tough to deal with.
When you get something not so good, you don't lose too much, and when you get something good, it can win you game, so on average it's pretty scary.
(As for why it's better than Far Sight, I think Far Sight both costs 1 more (which is a big deal), and Far Sight requires you to run the expensive card you hope to draw with it. With Unstable Portal, you can put it into a low curve deck, so it keeps aggro decks more streamlined.)