You are not forced to use Viking Units....

[Fatt_Shade]

LOL, you're joking, right? Thorns deal more damage then Black Dragons, per leadership of course.

Yes on paper, but consider att/def and resistances ratio of you and targeted enemy unit.
Expl : 1 black dragon and 315 thorn warriors attacking knight stack. BD will make 110-130 fire dmg with 70att so to knight with 27 def and 30% fire res it goes to average 120*(1+(70-27)*3,33%[this is increase of 133% for att-def])*0,7[this is 30% fire resistance]=204 dmg + possible burning effect
On other side 315 thorns 4 att, 1-3 physical dmg will do to same target 315*2(average dmg)*(1+(4-27)*3,33%[this is reduction for lower att then knight def])*0,7[this is reduction for 30% physical dmg]=280 dmg.

So thorn do make more dmg, but considering if knight stack retaliate they will kill some thorns, but hardly BD and next turn BD make same dmg, while thorns make less caused to loss on retaliation. And so on... Crit% is also factor here, but also is fact if you have 4999 leadership you can have 625 thorns, but still only 1 BD and then thorn will make much bigger difference in dmg uotput. Pure dmg/leadership ratio isnt only thing to consider in planing your army.