For my answer, its quite simple. He asked for the Best sub job. Its NIN. He didn't ask for a bunch of Situational Sub jobs.
in my opinion, /WAR is Situational Too, so is /SAM. i use /SAM when i fight Shinryu on THF when i go to Brew with my friend, So i can Sekkanoki before engaging and get 2 chances for a WS hint. or /DRK for Glavoid, But i wouldn't bring up either of those as the best sub job.
Thats why people aren't recommending /DNC. /DNC has its uses, its situational over /NIN though, for most of Endgame Activities involving 2 or more people, Your best bet is /NIN.
right now though, /NIN offers 5% more "Speed" over /DNC (i say 5% not 10% cause you could use Haste Samba). Which offers you faster kill speeds, etc. Also all the Debuffs on DNC are kinda weak, at best, and using them takes away largely from your over-all DPS due to the delay between Attacks and Job abilities, which gets worse as your haste levels rise.
Really though, Its because of Shadows, /NIN is ideal for shadows, There are some enemies where you just dont give a Fk. Like say, Glavoid for shells, but in that situation you're better off /DRK and just face-tanking and help stunning the big stuff like Gorge/Disgorge, especially if you low-man like i do.
It really breaks down too /NIN will be the most useful to him, its the "universal "can't go wrong" Sub job. /DNC, /WAR, /SAM, and /DRK are all Situational Sub jobs that should be maintained.
He has /DNC maintained according to his OP, So he's in the clear.
So its not that /DNC is useless, its that he asked for the best sub job, not the situational ones, and if you only got to chose 1 sub job to level ever, You'd pick /NIN, would you not? We (At lease I, Acknowledge uses for multiple subjobs, however i maintained to the confines of the Question asked, regardless of semantics.