What does a DRK want in a party?

[Phoenicia]

4-man: competent people.
8-man: competent people.

^ Pretty much.

On a side note: Why would you want a NIN to apply the slashing debuff when YOU, the WAR, should be applying it?

1. Ninja's Dancing Edge is the lowest potency out of the three options they have.
2. Storm Edge and Butcher Block combos are WAR's highest potency combos.

Also if you have a NIN he can Goad you without having the MCH sacrifice its turrets pulse AoE for your TP regen.