"What follows is a brief history of events since the Hellstorm, concentrating on the differences between this world and the path that history might have taken had the Hellstorm not occured. This world is a 'close parallel' of Earth; any event that isn't mentioned (e.g., the Korean War, the assassination of Martin Luther King) is assumed to have occured as in our world, perhaps with minor changes."
- David L. Pulver, GURPS Technomancer
You wouldn't want to imagine what's been lost, and gained, throughout history for want of a horseshoe nail. Lucky breaks like the Battle of Midway, where the US divebombers happened upon the Japanese fleet when it was at its most vulnerable, or the fickleness of nature, such as the Kamikaze of 1281 or the Protestant Wind of 1688, have influenced the course of history in ways subtle and gross. The present is built on an extremely brittle foundation, so much so that if the past was not fixed, nothing would be safe. We're all subject to the howling winds of happenstance, even though we've become so used to them that many of us can't even recognize them at all.
Alternate history is the act of tacking against the wind, of realizing the degree to which we've all built our houses on the sand. As I've said before, history is at the root of all the problems and prospects of the present day, to a degree that's easily unrecognized or glossed over.
Veteran sf author Larry Niven has said that he doesn't like writing parallel-universe stories, because in his opinion, in a "world" where everything happens, no particular event matters, and nor does the effort or sweat expended over bringing any one success to fruition. My own problem with parallel-universe stories, and with the genre of alternate history as a whole, tends more toward failures of creativity and invention. If something happened one way in actual history, the opinion in a great deal of published AH seems to be that there's no reason it shouldn't happen exactly the same way in an alternate history, even if the point of divergence between our history and the alternate history predates it by decades.
This is particularly common with historical characters; there are plenty of alternate history books that include historical figures in their cast who were born long after the point of divergence. Adolf Hitler shows up as an aide to a German general in Harry Turtledove's Timeline-191 series, even though the specific point of divergence - the security of Special Order 191 - took place twenty-seven years before he was born. To compound this attitude, Turtledove's The Two Georges not only includes Richard Nixon as a Los Angeles steam car salesman but makes Martin Luther King the Governor-General of the North American Union in a history that diverged from ours in the 18th century.
What's the problem? Only that acts of cross-historical pollination such as these greatly compromise willing suspension of disbelief. To suggest that mass movements and guiding philosophies are inevitable is one thing, and on the macro level is defensible - Jared Diamond's thesis that Eurasia was likely to achieve world dominance due to the nature of its geography is a sound one, even if there's no reason the precise shape of its dominance would have remained the same given a divergence far enough back in history.
But to introduce historical figures into an alternate twentieth century that is alternate because the American Revolution or the American Civil War went another way speaks of two things. The first is arrogance - "Martin Luther King Jr. was so important that he would have found a way to have been born even had the Roman Empire never fallen" - and the second is, to put it bluntly, laziness. Works of fiction are already filled with characters who did not exist. To me, using historical characters in a setting where they have no justification for existing doesn't provide verisimilitude, doesn't underscore the connection between that timeline and this one - no! What it does is jerk me out of the narrative and remind me that it's nothing but a work of fiction conjured out of a keyboard in California.
Even Robert Sobel, who with 1973's For Want of a Nail laid the foundation for alternate history to climb to new heights in the 1990s and 2000s, didn't entirely escape this, though the world he created is head and shoulders above all others in this regard. Familiar historical characters are mostly limited to the years immediately following the 1777 point of divergence, and then only because they were already active at the time. Subsequent to that, the only significant actual historical characters who appear in this 441-page alternate history textbook are Abraham Lincoln - himself a subversion of the trend, who is just an ordinary lawyer and is not even mentioned in the index - along with Sergei Witte, who in our history was Prime Minister of the Russian Empire and in Sobel's overthrew it, and Thomas Edison, who in both histories was a renowned inventor.
What really gets me is that, while the presence of Lincoln, Witte, and Edison are really historical in-jokes, their presence is like installing screen doors in a submarine: they don't improve the way the narrative works, and their very presence diminishes its ability to do what it's supposed to do, to be believed.
There's no reason an alternate history writer couldn't, and shouldn't, use historical characters born before the point of divergence and examine how changed history has changed them from what we know, or create her or his own figures of power and glory to fill the vacancies created when our renowned forebears were never born. Of course, the alternative would be - gasp - people actually learning about a previously unfamiliar aspect of history, and we can't have that, can we?