I understand Tolkien was trying to write a myth, but I really don't need to know about the Hobbits taking a bath before crossing the Brandywine river and whatnot. And I understand it's okay to go into more detail in a book, and I'm not saying the books are bad, or that Tolkien got it 'wrong'... just that it is more dramatic to condense and show the most important details.
The elves at Helm's Deep, I think, were basically a way of evening the odds. The idea that 300 men, half of whom were too old or young to effectively weild a sword, could hold off ten thousand orcs strains suspension of disbelief. I'd rather Jackson make book purists irate than lose the audience on something more farfetched than inspiring. Helm's Deep is not Thermopolae, and the people of Rohan are not Spartans.