mercs can fuel you early rushes/blitzes well

just make them fight and die fast.

they cost half the regular cost to hire and are instantly available but cost double the upkeep.

therefore, use them in wartime and use them fast for cheap overwhelming force.

merc druzhina should always get hired as they are 60 men FFKs when dismounted.

amulghavars are another that I always hire.

if you have a province with only an inn surrounded by enemy provinces, you'll have a gazillion mercs.