With skill and saved coin from past work, comes better arms and armor as well as something many a farm boy dreams of, a horse of their own to ride gallantly into battle on. While they are far from the most formidable men in Pendor, Mercenary Horsemen serve well as "plugs" to fill the gaps in one's cavalry forces and if really needed, extra garrison troops. All things considered, they make for decent enough light-medium cavalry for their cost and are well worth it to help fill out one's ranks until you train better replacements.
Strangely, they ask for lower wages compared to the bladesmen whom they often fight beside, but come the time where they wish to call themselves Sellsword Horsemen, they certainly pick up the slack on their wage demands.
These men form the bulk of The Free Company's cavalry compliment and should do the same for you if you plan to field an all mercenary force.
The Mercenary line leading to the horseman have been changed in version 3.7, which means they are now more costly to field and can be upgraded into the Mercenary Sergeant instead instead of the Sellsword Horseman(this troop no longer exists).