Oh, I believe you. I find it impossible to just refer to primary sources in things as relatively simple as the Korean War, let alone such a major history. And funny you should mention that, I just finished a decidely Euro-centric Short History of the World yesterday.Of course. Although sometimes there is a practical limit to what you can do. If you're writing the Complete History of Western Europe from the paleolithic to the present day, you can't possibly check primary sources for everything. That doesn't mean you just "cobble together" what other people have said - you must still exercise critical judgement - but it can't involve just referring everything to primary sources or you'd never finish it. Believe me!
What's annoying is when you borrow a book from the library, sit down to read it, and find that it's garbage, yet you have no other major sources to hand and an essay due in two days' time. ARRRRRGH!