This card really boggled me at first. I have to chain two attacks together to get a card that's nearly the same as Militia except I trash two cards and draw two more to replace it? Seems a bit better than Militia to start, but then you run out of things to trash and/or you have too many actions and the terminal card draw isn't very effective. Combined with being unable to actually get one early on, it seemed like it was completely useless until I realized the circumstance in which you're likely to have more stuff worth trashing and have a decent shot of chaining two attack: a cheap junking attack.
With Urchin + Amabassador/Young Witch/Sea Hag, you have a decent chance of connecting them Turns 3 or 4, and buying more Urchins instead of Silver will give you more speed through your deck to find your attack and give you better odds later on. At some point you'll probably manage to land a Mercenary and have a card that does all sorts of good things: trashing junk, providing money, and continuing to push your through your deck to find you attack. Unfortunately, there's the possibility of drawing your other attack dead, but being able to trash 2 cards while getting $2 will still be pretty nice. The attack portion of Mercenary almost become irrelevant with all the junk that might be floating around, although it makes it much harder to play one of the cheap junking attacks and still have enough money to buy anything.
Combining Urchin with a $5 junking attack seems much more of a stretch; if you're opening Urchins instead of silver it's going to be hard to hit $5 turns 3 and 4. As your deck gets filled with more junk, connecting the $5 attack with Urchins you buy later will be a bit harder and it'll be even longer before you manage a Mercenary. In the meantime your Urchins are much weaker economically than Silvers, and the attack is almost irrelevant against a junked-up deck; while you're going through your deck a bit faster, being able to hit $5 in order to buy more attacks seems like a better way of playing the attack more often.
Another attack Urchin might be interesting with is Minion, with the latter really liking good trashing and money from actions. Unfortunately, everything else seems pretty weak: You're going to need to buy a Silver or two in order to afford your first couple Minions, so you'll have to pick up Urchins later, probably with some sort of +buy after hitting $6 or an early $8, adding another you need to buy. Without villages you'll have to draw the Mercenary at exactly the right time and you risk drawing Minions dead, so that's yet another card you need to buy (if one's even available). And perhaps most damning, playing Urchin beforehand nullifies the effect of Minion's attack portion.
Of course, I could be totally off with my predictions, but taking a broad view Urchin/Mercenary looks to be quite narrow in applicability. It's possible that it's decent just stacking Urchins, but you probably need a very good reason to do that. Taking a more narrow view, it does appear to work better combined with the rest of Dark Ages. You'll want more trashing than in previous editions, and the discard attacks might also be considerably better as other people will sometimes rather hold on to their junk so they can trash it. This is the "discard your junk" attack of the set (Pillage being a much different kind of discard attack) so it likely needed to be tweaked from Militia, incorporating on-theme trashing as part of the attack.
