My google alert sent me towards an interesting post about the usefulness and validity of Alternate History made by the Grouchy Historian:

Alternate history
is a rapidly growing genre of science fiction that is finally beginning
to gain some respect and commercial success.  But, it remains less than
accepted by historians as anything but a fanciful flight of
imagination, useless for anything but passing some time.



However, your Grouchy Historian thinks differently.  I believe that well
written and thought out alternate history could be a very useful
teaching tool.  Now, I'm not talking about dragons in the American Revolution or anything here. 
Even I know ridiculous when I see it.  No, what I am talking about is
serious historical thought on what could have happened if a single
event, person, or action was changed--and how that change could have
impacted downstream events.



What do I mean by this?  Well, as a part time tutor and full time
curmudgeon, one of my major pet peeves when reading nincompoops talking
about historical events is the ol' theory of inevitability--history had
to happen as it did because of unstoppable forces, whether economics,
social pressures, or bigger armies.



Indeed, military history is usually the worst offender of this type of
analysis...of course the Confederacy had to lose, too many damnyankees
to kill, or, of course the Allies won World War II, because we had more
tanks, guns, soldiers, and ships than the Axis.  These sorts of
assumptions really drive me up the wall.  There is nothing inevitable
about history, and we should thank our lucky stars that so often history
DID turn out the right way.  AHHH, you want examples....here's a little
one that I will expand more later when I talk about my favorite
thing.....bacon....no really, books....and bacon...hmmm, eating bacon
while reading books...hmmm.



OK, think about the Normandy invasion and the battle of Omaha Beach. 
The German defenders, completely missed by Allied intelligence, nearly
swept the US troops of the 1st and 29th Divisions back into the sea. 
NOW, if there had been just 1 more German battalion in those
fortifications...or even if the Germans had been able to muster a single
Panzer regiment...thank how the Normandy invasion might have been
different with two widely separated Allied beachheads.  Would the Allies
still have prevailed?  Or would it have been another Anzio, where
Allied troops were penned into a beachhead with nowhere to go?



I think alternate history is an outstanding teaching tool to force
students to consider decisions NOT made, in addition to decisions made
by people throughout history.  Again, military history is the prime
example of this technique as our ol' friend Clausewitz points
out--generals must often  make snap decisions in the heat of battle,
with incomplete or outright erroneous information and it is often the
smallest quirk of fate that leads to decisions made and actions taken.

You can read the rest at the Grouchy Historian's blog (first link in the post). He elaborates a bit more and also recommends some good books on the subject.