The speed skill was pretty useless in Oblivion since you had a horse..now that it has been removed I think horses would be kind of necessary.
I wouldn't say it was useless, considering that horses couldn't be used indoors, in cities, or in combat, they could allow you to travel between places more quickly, but once you got where you were going, they wouldn't help you much. And often, I didn't bother to ride my horse anyway as it was annoying to have to constantly dismount whenever I'm attacked.
But whether the absence of speed will make horses important or not really depends on how fast movement is in the game, and how far you need to travel.
As to whether horses are important to me, I certainly would not complain if Bethesda could do mounts and do them well, but if they're not in, I wouldn't think of it as a big loss. Regardless, even if horses aren't ridable, I'd say having them in the game just as animals would still have its benefits, because it's not like horses don't exist or aren't used in Tamriel, it's just that riding them may not be part of the gameplay, and considering we saw a horse in the trailer, I think we can at least expect to see them, if Bethesda wasn't confident horses would be in the finished product at all, I don't think they'd let one make it into the trailer.