Easterling captains

What is the function of easterling captains? I can see a use for cataphrakt captains as they have 1 more defence than a mounted Dragon knight but in the case of captains, they dont.

Captains have -Possibility to support (when do you actually do this?) -1 more faith -can take non BD units up to 12 -march (but this is rarely usefull becouse of wardrum possibility) -10 points cheaper

Dragon knights have

• +1 attack -Strike
• +1fv
• Shielding -Ability to take BD:s -Gives +1fv to BD:s (very critical, especially of you are using amdur as cruise missile)

I just feel that it is very rarely actually worth it to take captain over DK, the extra killing power and hero killer threat that DK provides. I even think that DK has more survivability compared to a captain due to shielding, higher fv and attacks. The only reason I see to take captain is the extra unit slots but most of the time you play in points where you cant afford more units either way or you have plenty enough?