I always found the ranged attack potential the marauder horsemen have to be sort of a moot point. They're pretty much it in the army, and they're not particularly good-- why bother trying to make up for a weakness when you can play further to the army's strengths?
With that in mind, I like flails on marauder horsemen. Doesn't matter what mark they have-- they're cheap, they're quick, and they're WS4 and S5. They're great for sticking a big horde in the flank so your Warriors can break through and move on to the next target. Really, it's the only way I like running Marauders-- while Marauders on foot are a great buy, I've found they still end up outclassed by other light infantry just because you can't get as many in base due to their 25mm base size. If they were on 20mm bases, they'd be a lot better, I think. Since they're on the same base size as Warriors, and one Warrior is more than three times better than three marauders (which cost the same when armed with a flail, great weapon, etc), I've found I just get a lot more mileage out of units of 14-19 Warriors lead by a hero instead of the gigantic mobs of northmen.
Anyway, the Marauders can also go hunt down wizards and war machines the way warhounds used to be able to. Five Warhounds aren't an effective screen or quick hunting unit the way they used to be, but you still need something to go deal with bolt throwers and cannons as fast as possible. The horsemen can do that and then turn back around and continue to support the rest of the army, where a sufficient unit of warhounds gets depleted doing the same thing.
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