This is true but with a Sonic/* defender you don't really need the 4% added resistance to cap S/L and for FF/* defender you can have soft-cap defense to everything... Add in 68% resistance to S/L and you're already at tanker level protection from S/L. Granted, less hp, but not even a lot of tankers will have soft-cap defense AND 68% S/L resistance. Have 72% instead isn't going to matter too much.
But having 32% resistance to psionic or energy or 22% resistance to neg & toxic can come in handy more often than you think. Admittedly the last one not as much, but Dark Mastery has some other things that a "tankmage" can find useful.
Because of this thread I'm working up a build for a FF/sonic/dark defender. With opp. gloom and screech I'll be able to stunlock bosses, have tons of defenses, and enough resistance debuffs to add something to the party's overall damage while boosting my own fairly well.
I may never play this toon but it looks damn fun.
In teams: Howl, Screech (boss), jump in with OG on, Soul Drain, blast as desired.
Solo: Siren's Song, Screech (boss if any), jump in with OG, blast as desired.
VERY safe, reasonable damage, and with Dark Consumption there's not an endurance crunch.