I think we'll need to see what mechanics they give the enemy riders before saying with confidence whether or not they'd be useful on our side as well.
That said, even if they are just really fast moving, I can imagine mounted units being useful depending on if they extend the combat orders mechanic a bit. Like if you can give split orders to different unit groups, you could have most units set to attack the enemy melee units and the riders attack the enemy ranged units while the melee are tied up. They did previously have a way to group units before, although admittedly for a different mechanic, so I don't think it's completely out of question.
I was thinking orcs would be the only monsters we've seen so far that wouldn't look goofy riding horses. Goblins seemed a bit too small, but if they add wolves, that would certainly open up the possibilities.
I do like your idea regarding raids being sped up too though.