Unnamed sources are often critical contributors to important news reports and, as I have noted before, Trump has no problem with them, when he finds their disclosures helpful.
But anonymity invariably promotes skepticism about sources' motives. The Times wrote that "all of the current and former officials spoke on the condition of anonymity because the continuing investigation is classified." That makes sense; it also makes sense to wonder whether these officials have political agendas and to consider what they might not be revealing.
See my previous post for more about reporting rumor as fact.