It's certainly better than treasure map though, since you can usually get two on the first two turns, and the cantrip effect increases the odds of them colliding a bit. Someone fresher on their math can figure out the actual probabilities. Plus the urchins themselves aren't dead cards, they replace themselves and do a minor attack. Warehouse, tactician, and the like can accelerate the collision, but trashing is counterproductive to actually using the mercenary
Whether the reward is worth the risk depends on what's in the kingdom. If there's no other trashers on the board then getting you junk trashed is a huge benefit by itself. Mercenary is militia + steward with all three options, or like masquerade + militia + more, so it should thin your deck without slowing you down better than anything else. Cursers will keep your mercenary viable for longer. Even if you only get to use it 3 or 4 times, you've now got the trimmest deck, while everyone else has been militia'd a bunch, which should more than make up for whatever they opened with that was better than urchin.
I suspect I'd go double urchin with warehouse, cellar, or scheme on the board given no other good trashers. Also, as a swingy opening it's always good when you're already behind, like second player with 4-3 vs. first player with 5-2 and witch on the board.
