Chapel is useful for getting to your power card more frequently (e.g. opening Chapel/Witch) and for getting your actions to cluster together. That is, if something like Village were on the board, then Chapel would help you chain together your Villages and Margraves.
Chapel has at least two downsides though. First, it requires 2 or 3 turns of trashing during which you cannot buy anything. Often you can make up for this set back by having a higher density of good cards. But second, having such a slim deck, you'll really start to hit a wall once you start buying Victory cards, since 2 or 3 Provinces will comprise a signficantly larger portion of your deck post-chapel than it would have if you'd kept those 10 original cards and been buying Silvers. For this reason, a Chapel deck often wants to build up to the point of being able to gain multiple Victory cards in a few quick bursts at the end. Assuming that you've just been hit by a Margrave (your Wife is playing a multi-Margrave Chapel deck, after all) but then play a Margrave at the start of your turn, you'll have a hand of 5 cards. Even if all 5 of these are Gold, you won't be able to buy two Provinces at once. To even get a Province and Duchy, you'll need $13, which means 3 Golds and 2 Silvers -- quite a feat indeed, particularly when odds are high that you'll draw a dead Chapel or Margrave, and even worse after your first round of greening.
