While not as mobile as normal light cavalry, Camel Warriors have several advantages which make them somewhat ideal as a counter against cavalry-heavy factions. The first and most obvious perk is that they have a defensive bonus against most melee cavalry units, thus making them more effective as a defensive unit attacking cavalry. The second is that despite being slower, Camel Warriors have substantially higher hitpoints and armour compared to normal light cavalry. Finally, camels don't have so high a ramp cost in food as horses normally do, thus meaning that you can raise many of these units with little ease.
That said, as melee units go, Camel Warriors aren't the best units. Although being cheaper and tougher, they are slower — both in movement speed and their attack speed alike. If you are facing an enemy who is training many camels, it will serve to have heavy infantry like Hoplites or Roman Triarii in your armies and fast-moving missile cavalry to counter them. Even simple cavalry units like the Gaisaredos or Equites Speculatores can be used to effectively counter them, by virtue of their dexterity.
Despite being slower and much less temperate than horses, camels had their uses. The first was their durability — on the march, camels can store all the fat and moisture that they will ever need in the hump on their body. The second was that their scent often spooked horses, which was both a blessing and a curse for the military-minded — it caused problems for enemies on horseback, but it could also cause problems if handling baggage, and keeping an army of camels and horses apart could sometimes pose problems. The earliest known instance of tactical exploitation of a camel's scent dates back from the conquest of Lydia by Persia, where the Persian Shah Cyrus deployed a detachment of camelry to spook his enemy's cavalry. When desert warfare experts were required, camel-riding nomads could be recruited as mercenaries, as was done in the wastes north of China, or in the arid sands of Africa and the Middle East.