So. First off, it's three chapters and a synopsis, not two, as the standard. Secondly, it's extremely unlikely that you'll actually sell a first novel based solely on the above -- what you will actually get, if you're lucky, is an editor who is interested enough to want to see the whole manuscript. Which you should then have ready to give them immediately; they'll expect you to, if it's your first novel.
I think part of the confusion that arises here is simply a distinction between the process for first-time novelists (i.e., people who are hoping to publish their first novel) and established novelists. Once you've sold something and had readers buying that book at a reasonable rate, a publisher is much more likely to trust (gamble on the hope) that you can keep doing that, so they *may* be willing to buy a book from you based solely on synopsis/three chapters (or less). It's risky for them -- they're paying out an advance for a book that may never be completed, and if you don't finish it, they never get that money back. Lots of writers don't finish their novels. So it seems reasonable to me that publishers would be a bit hesitant with advances to unproven writers.
This applies if you're crossing genres too -- even if you've published a kazillion mysteries, you can still be a first-time sf novelist. For example, I was talking to Duncan at Melcher the other day, and he was saying that they were doing some agenting, so if I had a mainstream book I wanted to sell, they'd be interested in looking at it. But since I don't have any publications yet in mainstream or young adult lit., they'd need to see a full novel before trying to sell it. Whereas I have enough of a track record in erotica, that he thought I could probably sell a book on synopsis alone at this point -- they would trust, based on past experience, that I could write a decent erotic book based on my synopsis. But basically, these last two paragraphs you should only worry about after you've sold a book or two. Until then, you're not going to sell a book based on part of it -- you're just going to get asked for the rest of the book so they can make their decision.
The *only* reason publishers do the synopsis/three chapters thing for first novelists at all is due to sheer lack of time (and hell, it saves you postage). Editors at publishing houses don't read manuscripts during the work day, you know. They develop the books they've already bought during the work day, and it takes the whole day, every day. Reading manuscripts happens on the train home, late at night when the spouse is asleep, on the weekends. They read manuscripts from unknowns because they love books and they hope they might find something amazing. But they know that most of what they get isn't amazing. They know, based on experience, that there are far more first novels being written out there than they can afford to publish. They know that most of them are utterly mediocre. The synopsis/three chapters thing is just a first step, a winnowing process.
And publishers do know that some novels don't start off with a whiz-bang hook -- I think the advice people give to always start with a hook is a bit misleading. You don't need a hook. What you do need is to give them some reason to keep turning pages. You need to catch them with something -- be it the coolness of your ideas, the beauty of your prose, your insight into human character, a mystery suggested in the first lines and left hanging...something. Something that draws them forward, past the first line to the first paragraph to the end of the first chapter, to the end of the second and third chapters, so that they're then calling you up and saying "Hey, I'm really interested here, send me the rest of the book by Fed Ex." And you say, "Sure. It's on its way." That's how you sell a first novel.