Honestly, I think that party movement would be awful if the entire group were to be mounted. You know those times where you're standing around and all three party members keep dancing around, step left, step right, step up, step back, to get in just the right position so they are arrayed in perfect harmony around the PC? Imagine that on mounts... That would be horrendous, and absurd, and absurdly horrendous.
I like the idea of mounts, but I just don't think they fit well in these games.
That said, I think if they are going to have mounts that they should give us more incentive to use them. No, not by having ginormous maps -- I never used a mount once in the Hissing Wastes -- but by having mounted action be a part of the game. I'm not referring to combat, but in requiring mount use to get somewhere, more use in cutscenes a la Trespasser, perhaps have mounts be the way to solve puzzles. There can also be more meaningful dialogue, followers referencing mount use, perhaps interacting (petting, feeding) with mounts at stables or after dismounting, and so on. As silly as the Skyrim mounts were sometimes, I did like the idea that they just stood there, waiting, until you returned; a despawning mount is rather immersion breaking.
As for the mounts that we have, the movement is a little clunky and awkward. Horses don't turn or stop on a dime, but they are a lot more agile than the DAI mounts present them as being.
Also, while I agree that variety is nice, and I certainly don't wish to take away anyone's nuggalope or dracolisk, it might be nice if the player were encouraged to form a partnership with a single mount. The mount can certainly be one of your choosing, but bonuses might be given for continual use or something. History is full of military leaders who had affection for their loyal mounts, and this is something I'd like to see as well, rather than the player saying, "Oh, I feel like Bog Unicorn today and the Hart tomorrow."