I'm playing one of these guys in Pathfinder too, Halfling Beast Rider/Emissary Knight Errant Cavalier / Warrior of the Holy Light Paladin. It's a mouthful, but he's riding a wolf, which invalidates all arguments against him, haha! Your guy here serves as one of the inspirations for my character, when I get around to drawing him.
Faster movement speed, two targets for enemies to pick between, another attack, and in the case of a wolf in Pathfinder, it can trip them for you, allowing your attack a +4 bonus... also more carrying capacity, and you take up the same area as mediums do, and often get size bonuses to AC. There's lots of benefits... but some downsides, too, like dealing damage a size category lower. Magic helps though, as it always does.
I saw this piece on a random Google search months back that inspired me to create Milo Merryweather, a boisterous Halfling paladin, for my D&D group. He has since passed on, but he was wonderful to play. Thanks for creating this inspiration. Your artworks has a lot of character which "draws" me in. Pun intended.