Maybe a horse expert can correct me, but I remember reading somewere that infantry units could march farther in a day the calvery could ride without killing thier horeses. The speed advantage of the horse is for short sprints. Keeping them together is probably the best bet. Remember Strider (with his broken sword) & the Hobbits chase off the 5 BRs on Weathertop. They would have run the risk of meeting the Nazgul alone with a winded horse. Besides the rest of them wouldn't have been able to scare the black horses into the river if they were left to far behind.
Since the movie left alot of that out the 'chase scene' to the ford worked much better.
I think on the whole I am OK with the changes that Jackson made, though I didn't think Arwen's increased role was necessary, I didn't mind it either. In fact, her scenes with Frodo were very good.
Jackson's (and Alan Lee's) version of Rivendell, however, was not exactly the way I've always imagined it. I didn't picture it to be such an "outdoor garden" sort of place. Somehow I imagined a large house with cozy fires and hearths and stuff. I didn't picture it to be in such a narrow chasm either. Not that I didn't like it, just that it was different than my own vision.
The falling leaves in the Rivendell scenes were a thoughtful touch. I know, in the story, the season was fall, but the falling leaves also very eloquently and poignantly foreshadow the end of the elves time in Middle Earth.