It's almost impossible to trace how rumors get started. There are agents with differing opinions, agents who publish books to make money, people who are NOT agents publishing books about query letters to make money, and the like. The more generous side of me thinks that at the beginning of this game of telephone, some agent or assistant was attempting to give out good advice about a specific situation, and too many people took it the wrong way.
Bad Advice #1 - "Discuss your potential audience."
This only applies to full non-fiction proposals (the ones that are formatted to go on for pages). It does not apply to query letters for both fiction and non-fiction. We know who your potential audience is: people interested in the genre/topic you are writing about. Books have built-in audiences. That's basically the reason that books are seperated into genres - so people can find what type of book they're looking for more easily in the bookstore than if every single author was just listed alphabetically.
I've seen this taken to stupidly hilarious levels, implying that either most of or all of the population is going to buy and read their book. Three examples, in order of stupidity, of things I've actually seen in query letters:
3. "My detective protagonist lives in an RV home. 4.2 million Americans own RVs, and I think this book will appeal to them." [Yeah, I'm more likely to buy a novel that has a main character living in the same type of home as me. That's a definite sell right there.]
2. "The man who must stop the terrorists is an airline pilot and the novel takes place on an airplane, so it will interest people who have seen or ridden on airplanes." [I've been on an airplane! I can immediately relate to this guy! Sold!]
1. "The protagonist is a woman, and I think that will appeal to the female population. However there is also a male love interest, so the book will cross gender barriers and also appeal to men." [What about transgendered people! Huh?!? That's a big market!]
The only thing you're telling us when you talk about your potential market is that you did some bad research on the internet that told you to do that. It doesn't mean it's an auto-reject, but it doesn't help your case.