That no epic movie based on historical events/people is going to be that accurate. (even if one was, how could we prove it was?)And it doesn't usually bother me too much... Rome (HBO) was my favorite television series EVER but only after I decided to enjoy the show for its entertaining qualities and not to disect it for its historical misdirection.
But the reason these flaws in cinema have always bothered me, is because I've seen WAY too many people watch a movie like 300 and then instantly start lecturing everyone about the Spartans as if the movie were an educational seminar... it's frustrating and I'm sure you've all heard it as well. But this is not the fault of hollywood- it's the dumb audiences that hollywood caters to.
However, I do LOVE The Killer Angels (based on the novel which is excellent!) I know it's not a swords and sandals epic by any stretch. But it's extremely moving and, as far as I can tell, as accurate as any other account I've read on the battle of Gettysburg.
Of course, it's much easier to make such a movie about an event that happened a mere 145 years ago compared to movies like Gladiator or Braveheart.