Like many agents, the practice of using a rhetorical question to open a query letter rubs me the wrong way. I've never really been able to pinpoint why, but over the past month, I've heard ample pitches and read ample queries to allow me to figure this out a little better.
Many of the pitches I heard began with the rhetorical question that probably all authors ask themselves when they begin to write: "What if?" What if spaceships landed in the Himalayas and aliens infiltrated a Buddhist monk colony? What if a woman was charged with murder after the accidental death of her husband? What if Rachel Zoe was actually a zombie? What if The Breakfast Club took place on Antarctica? (In case you were wondering, these are not pitches that I heard.)
This question and the wonder that follows can be the root of inspiration for a story--the knot the author seeks to unravel; the beads the author worries into a plot. So the "What if?" can be a rhetorical question that the author asks herself. It doesn't bother me as part of a pitch. An in-person pitch has more room for conversation, after all, and rhetorical questions are often used in conversation. That being said...
The rhetorical question doesn't work as well in the query letter. I think it's because, in writing, it comes across as a lazy device. This book--an entire manuscript, thousands of words(!)--has been written by you, the author, and has been plumbed from the depths and heights of your imagination. One of the reasons I love reading query letters is because they offer glimpses into that imagination. But using a rhetorical question to start a query letter takes the onus of imagination off of the author and places it on the reader/agent's shoulders. This is a risky practice. It takes an agent out of the query letter and away from your story, when your query is your one chance to showcase the writing and imagination that you poured into your story. You'll be hard-pressed to keep a rapt audience with rhetorical questions, and you need a rapt audience to get the attention your query letter deserves.