Pretty much if you are a DRK in a dungeon you want a SGE or a SCH with you. Specifically SGE due to dedicated sustain coming from Kardia every GCD and things like Haima every other trash pull. When you get a WHM or AST if they run out of OGCD heals then good luck getting a GCD heal from them.

Honestly DRKs whole dungeon problem could be fixed if they removed AD from CS. Put AD on the GCD with 30s cooldown with 2 charges and make a low level TBN ( call it shadowskin) at lvl 40. That would greatly improve the leveling process and improve expert dungeon runs on DRK.

But devs and JP think DRK is fine as is. JP only have issues with plunge being removed but that is getting fixed in 7.01 with emnity added to the dash. So looks like we are waiting until 8.0 for sustain fixes if any.