Historical fiction: How to judge it, what value does it add?

As expected, by the end of part one of E.L. Doctorow's The March two things happen: the various strands of plot entwine as the characters meet, cross paths, realign, in the occupied (and liberated) city of Savannah, and, the novel becomes more of a conventional historical fiction, as Doctorow introduces historical figures as active characters in the story - General Sherman, and Secretary of War Stanton (maybe some of the other referenced minor characters are actual as well). Though I haven't read a lot of Doctorow's fiction, I know he's known for the blend of created plot and historical plot and incidence - establishing fictional characters who lives among and actually can affect the events of their time. I'm generally not drawn to that type of fiction, but Doctorow's narrative skill is exceptional and I found the scenes of carnage, military advance, and the life in a liberated city during wartime to be extremely vivid and powerful vignettes. It's taken a while but I also am now able to hold straight the sets of characters - he introduced many characters at first and some have fallen by the wayside while a few are driving the plot: Ms Stephens, daughter of a late judge working as an aide with a union surgeon; Pearl, a half-white freed slave girl traveling with General Sherman disguised as a union drummer boy; Coalhouse Walker and another freed slave who've decided to take up the union army offer of 40 acres of land and stay in the South; and two confederate deserters, Will and Arly, who pose as medical aides and travel north with the army, now heading into South Carolina. The question we face when reading historical fiction is always: what value does it add? Would we be better served reading a straightforward account of the historical events? To succeed, the historical novel has to fill in the headlines of history with the lives, feelings, beliefs of people living in the time, while also being true (though some writers are whimsically untrue) to the events of history: War and Peace being the touchstone for all. It helps if readers bring some basic knowledge of the history to the threshold: it's probably way easier for American readers to take on The March (and for Russians to take on W&P for that matter). This novel so far is very appealing to read - although I am beginning to feel that the narrative strands are tied a little to coincidentally and there's an element of Hollywood theatrics here as well - love in time of war, the buddy movie, oppressed man standing up against the system.