When researching for my novels, one temptation I face is to sample sources instead of reading them fully and to skim through them at warp speed till I find the parts that are relevant to my work. After all, who has time to read William of Malmesbury's whole Chronicle of the Kings of England when all you need is his bit about the Norman Conquest?
One important thing to remember, however, is that research isn't just about finding the facts put forward in a given source; research is about getting to know the historian. When we take the time to evaluate a primary source as a whole instead of picking through it like a flea market connoisseur, we better understand the thoughts of a person living during that time period. And surely that person (even if he only represents a segment of his society), has a better grasp on what facts, what anecdotes, what lists of information were valuable to the men of his era. Sometimes, the boring and bizarre bits that we would want to skip are actually the key to unlocking an understanding of the period.
One example of this is the convoluted Easter controversy in Bede's Ecclesiastical History. To the modern reader, this disagreement between the Celtic churches and Roman churches may seem like a molehill on the horizon of historical mountains. One might be attempted to gloss over the liturgical controversy and excerpt out the "important" bits about the politics and polity of the Anglo-Saxon kingdoms. But to Bede, this subject was of primary importance, and if we look at Bede as not just a scribbler of facts but also a window into his age, we learn what was important to the eighth century Britons.
Excerpts of primary sources can be excellent timesavers, but they can also be misleading. Excerpts give us the bits that the excerpter (is that a word?) thinks are important, not necessarily the bits that the author considered most essential. I'm all in favor of skipping ahead to the important stuff, but sometimes it takes a thorough reading of a work in its entirety to know what the important stuff is.