Originally Posted by
Callei
Sure, but those two pandaren established that they were fairly badass, which seems to have been the only entry requirement for the Acherus Death Knights (who were, it's implied, purposely undertrained and poorly-equipped so they would be just tough enough to keep Tirion busy until Arthas could make the scene; ditto for the thousands of Scourge he zerg-rushed Light's Hope with). Seeing as how Arthas never intended for the Acherus Death Knights to be a permanent fixture in his Scourge, why wouldn't he take advantage of two creatures with an exotic fighting style that would lend them an edge?
edit: If you're trying to argue from a player population stance, that's never really effected Blizzard's choices in lore before. The Death Knight starting quests treat it like there's only one Death Knight PC of any given race, with the rest of the Ebon Blade filled out by NPCs both named and unnamed--yes, on RP servers we're free to disregard the implications of A Special Surprise for more original ideas (for better and worse as the case may be) but as far as the actual lore handles it, you're the poor sap who killed his/her best friend at their behest when their speech didn't get through to you.