You're getting more than just one set of monster-laden dungeons, though. Labyrinth contains two separate games that share the same general setup, but two completely different storylines and overall appearance. The "Shawn and Ace" game is made for newcomers and, in a shocking twist, actually has a pretty interesting story running through it.
While on a family car ride, you and your family are sucked into the world of memories, where anthropomorphic elephants, mice and lizards either guard our precious history or are tasked with deleting our old, unwanted memories. Turns out, both of your parents hate each other, regret having you and just want to start their lives over. So, the monsters are just carrying out their jobs, deleting memories, but as a child, you can't accept that and press on, hoping to find your lost parents and stop them.
The other storyline, "Wandering Soul," is more or less a straight-up dungeon crawl and made for experts of the genre. In fact, this is a port of a cell phone game from years ago. If this were the only game on the DS card, we'd be much harsher, but as a bonus, "Soul" isn't that bad. You're looking at the same weapons, spells and problems of the first game, so its inclusion may or may not matter at all.
For a little extra street cred, the scenario for Labyrinth was cooked up by Masato Katou, who gave us a little game called Chrono Trigger, and the game's excellent music is from Chrono Cross composer Yasonori Mitsuda.