Well like most tier 2 units they should mostly be used to deter the AI from starting a siege in the first place.
Their damage is pretty pittiful agaisnt higher tier units, so you need to have a core of high tier units regardless. The issue also is that being archers they don't go towards the breach by default, so they're not as usaful as tier 2 INF that will clog up the place to hold the hordes while your high tier units do the actual killing.
Swadian militas are ususally a better option for tier2 filler as they have a chance of spwaning with a crossbow, which will do more damage at their level than bows. Can't remember if the tier 2 Nord INF unit have throwing weapons, if so they would be more useful IMO for a filler.