Should You Buy Dragon Age II's Downloadable Content?

That's the question asked by IGN in this editorial/review for The Black Emporium and The Exiled Prince, though their conclusion for both titles is that they're largely unnecessary. $10 for a merchant and a summoning ability:
The Black Emporium itself is a trippy place. There's a golem and a street urchin who watch you while you explore (or ogle a partially naked statue of Andraste) but never speak to you directly. It's sort of creepy and awkward. Anyway, you don't go there for the boobs or the conversation, the area is essentially a bazaar of weapons, poisons, tomes (including ones that add attribute points and one that lets you re-spec your character) and runes that you can't get anywhere else in the game. There's also a mirror so you can change the way your character looks in case you accidentally put on green blush or want blue eyes. While there's good stuff available for purchase, there's also a spot where some weapons and armor are just lying around for the taking. Loot is great, but it's still possible to get sufficient equipment from the core game itself, so these items are more of a luxury than a necessity.

If you don't purchase the game new, I don't see any reason to cough up 800 MSFT points (or $10) for it. I mean, unless you want a Mabari really badly.