To my understanding (and I may be wrong here) Fast-travel is a pre-calculated time based on distance between point A and B, assuming that path is a straight line. So your fast travel will be just as fast with or without a horse, regardless of the armor you wear and the gear you carry.

In general, and especially over long distances like across the map, fast-travel is faster simply because you skip over all the twists and turns and obstacles that lay in your path, as well as avoid combat with bandits and such. In the short distance, like say from the stables of Whiterun to one of the giant camps to the west, it would be faster to just run there, and faster yet on horse, rather than using fast travel...as least as I see it.

Again, I haven't done any calculations and I don't have any data to back any of this up...its just what I've observed so far.

lol There was a point to buying them before Arvak? I can hop off and run faster than horses in this game.

You can use them while over encumbered and mounted combat.

RP reasons. And for characters that doesn't do Dawnguard.

Fresh meat!

I actually don't like using Arvak. I think he looks quite awkward. Plus the one character I completed dawnguard with doesn't use magic. Just doesn't fit!

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Regular horses look much better than a blue horse skeleton.