I think losing ~5 turns buying and then connecting Urchins is a small price to pay. Compare with Chapel, you will spend about 4 turns on buying Chapel and then trashing with it, and it's one of the best cards in the game. Mercenary gives you $ while you trash, so later turns spent trashing aren't wasted, you can at least get a Silver, which is probably what you were doing in between Chapel plays. And as SCSN says, it's an attack, it slows your opponent down too.
OK, Steward can compete, I can't deny that - Steward is a pretty good card! And of course you should take Chapel over Urchin for sure. But other trashers that WW ranked similarly - Loan, Trading Post, Lookout, Forager (actually I like Forager a lot, I think WW underrated it too) - I'm never going to skip Urchin in favour of those.