Sourcebooks are a blessing for beginners in small or rebuilding clubs. It's great to have a model to follow when you're still struggling to master the basics. It's better still to apprentice yourself to a more experienced debater who can explain what she's doing at every step, but not everyone has that.
Sourcebooks become a stumbling block for intermediate to advanced debaters who make the mistake, not always conscious, of letting the books frame the topic for them, telling them what a topical affirmative really looks like, what their options are, and what strategies are winning strategies. They're much better off analyzing the topic, doing a lot of background reading, sweeping the library
, and then
using sourcebooks to discover pockets of the topic they'd overlooked. Best of all is when they use the sourcebooks only as compilations of cites, and re-cut every useful article themselves, because even if they cut the exact same cards, the work put in reading the paragraphs between the cards and digesting all the details that get boiled down out of the argument will make them a thousand percent more authoritative on those issues.
It's a lot more work that way, but the people who win consistently are invariably the ones who put in a lot more work. Several of my fellow coaches over the years have dropped the opinion on me that the winner in any particular debate will be the team that wants it more. I agree with that, but I don't think you best judge it by the conduct observable between 1AC and 2AR; I think the test of it comes in the pre-tournament, and even pre-season, prep.