I'm pretty sure it's not a must open.
What are you talking about? It gets rid of all of your starting cards immediately. That's like, all you could ask for from a trasher, right?
But it slows you down. For the sake of argument, let's assume Chapel and Donate are available.
Player 1 Donates in turn 1. In turn 2 he pays off $5 in debt, in turn 3 another $3 and a Chapel, in turn 4 buys sth for $5. So player 1 is left with 5 copper, a $5 card after turn 4.
Payer 2 buys a Chapel in turn 1, some $4 (or $5 card if lucky) in turn 2, trashes 4 cards in turn 3 and buys (possible) a Silver in turn 4. Player 2 has two Estates, the Chapel, some $4 card, a Silver and four Copper. (This would be with average luck, could turn out more beneficially or worse.)
So Player 1 has a leaner hand now, but still five Coppers, while Player 2 has two more cards to trash but an additional Silver, and a $4 to Player 1's $5 card. (Player 1 could decline the Chapel, but would be left without a plan to trash the Coppers.) I'd maybe open Chapel over Donate if there is a strong $4 card on the board and $5 cards are less than convincing, and seriously consider opening Chapel on a 5/2.