Sure, but it also leads to glaring omissions and intrinsic bias because, importantly, not everything is written down at the time it happens. It also assumes everything written down is honest and true, when that's not always the case.
You can verify various primary sources against each other. Obviously, a drawback of this style is that it is much more demanding of the researcher, and that there will be times when the record is unclear and you can't comment- and, again, I think Peterson does a great job of saying when the record is incomplete or contradictory.
To me, this is infinitely preferable to relying on people recounting stories
after more than forty years. Is it possible that the contemporaneous sources aren't correct? Sure. But the contemporaneous sources have the major advantage of being ... contemporaneous. Unaffected by the passage of time. Of accurately recounting events of that time- as opposed to what people say decades later.
Simply put, it is beyond bizarre for someone to say that there is
more intrinsic bias in using contemporaneous primary sources than there is in asking people to recount their
personal experiences from decades ago. But again, you can see the advantages of this when you're looking at, inter alia,
Game Wizards. It is much more instructive to see how Gygax and Arnerson wrote about each other
at that time than it would have been to get an oral history from them 40 years later, when both would have been motivated (to the extent that the accurately recalled all the events) to shade the history in ways that flatter each of them in the present.