The light cav path is useful for a number of reasons, but in the current state of the game you need to have different types of units to support each other. Once they have the 1 MP to pillage promotion they can win a lot of prizes, and heal damage. Their mobility also makes them great for providing flanking/support bonuses to other troops. They can always prosecute a siege but as has been mentioned they're bad at dealing with walls. Granted, Coursers CAN brute their way through Ancient Walls even without a battering ram, but the fact that they can't benefit from Siege Towers means that you need another answer for anything stronger than that. If you don't have iron but do have horses (which happens frequently) it certainly makes sense to prioritize them if you want to make war but they aren't the right tool for every job.