Horses have great use in-game - 1st of all hippus (you cant play hippus good w/o horses, all their powers is horse)..
The key is additional move and no penalties for city attack, aviability of weapon upgrades (compared to recon units)
They are really usefull, espesially with raider trait.
sidar, Elves, Dark Elves, Lanun are great races to use mounted units with.
The key is farming.
Lets say you have too much cities already captured and you wont anymore.
Then you can just farm some small neightbours.
Give them build towns, declare war and pillage.
With horse units you will be pilage 1-2 tiles more, that mean great income that can be critical for your game victory. (i managed to get pillaging income of 1000gp per turn

)
also strategically, and tactically (bring reinforcements fast).
Also withdrawal chance, which is also very good ability.
Ratha.
Empyrean is best race to rely on horse units building and horse army , cause it allow you having same move Rathas (+ same tech tree) , that can just blind enemy, so you can pillage and move where you want UNPUNISHED and not countered, just by using mobility 2 rathas that will blind all enemy units around, i mean ALL , so enemy cant bring reinforcements, move, attack your weak units, and do nothing., the key that if you have army of only horse units all with 5 moves, that mean you can pass 15!!!!! tiles obn enemy lands with engeneering (10 without). You can attack and pillage multiple targets and still at
end of turn you can gather all your army on same tile , in one, gathered strong stack.
Such armies can kill strong mage stacks and flee unpunished ( 15 moves, starting point have casters with all possible enchantments (haste, bless , etc ), and hit-and-run strategies.
at some point, when i found this strat i had though that hi-XP horse-only armies and raider trait is bit overpowered, but it is ok, and is good counter for mages/ strong slowmoving units.
Sidar.
My addition is Valor spell and ability of your army to gather in some starting point with casters (city, forest+ hill, ancient forest, castle) , so you can attack and then move your army for regeneration, valor recasting.
Sumarry.
If done well such armies can wipe any others + ruin enemy economy COMPLETELY within few turns.
Seems author just didnt tried to play Hippus or Raider Empyrean Horse armies, so i advice him to do so - it IS great fun.