You forget, boars have a rather NASTY pair of tusks that are capable of doing serious damage-so much so that wolves prefer to leave boars alone and go after other prey is there is ANYTHING else they can eat. And while they may live in moutainhomes, they have to leave them to go to war or trade-and it's terribly inefficient to have foot soldiers walking alongside a caravan. Much better to have cavalry guarding the caravan, so as to counter raiders/bandits riding cavalry. And by the way, dwarves can and DO get involved in disputes during world gen, and will be traveling overland once the army arc is complete. Elves can ride unicorns, humans have horses, goblins ride LIZARDS (!!!), and you say there isn't a dwarven mount?
The main advantage to cavalry though, is tactical mobility. They have the speed to perform maneuvers in battle that infantry simply DO NOT have. For instance, rapid hit and run strikes along multiple points of the enemy's flank-or better yet, their supply lines. Size difference? If size difference was an issue, dwarves would loose their battles with humans and elves because of it. They simply can't hole themselves up in moutainhomes all day every time they run into trouble, or they would be unable to grow and expand.
So in reality, cavalry of some sort strike me as pretty much a necessity. Especially if the enemy has heavily armored cavalry (knights) on their side, having a dwarven version would at least even the odds better. Crossbows are pretty short range compared to longbows, you know.