I don't think there is anything inferior about /dark as a mastermind secondary. Howling Twilight, Fearsome Stare, Darkest Night, and Dark Servant can mitigate a ton of incoming damage.
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.
My DS has Dark Miasma as its secondary, and I love it. I don't know on what basis the person who told you to go PD is working, but my first MM is nec/dark and I have a nin/pd MM - I find they're both incredibly versatile, but I play them very differently. In fact, I play each of my MMs differently.
Getting back to the OP - I agree Psy - there is nothind inferior about DM. It's a brilliant secondary armed with a strong heal, tons of debuffs, an AoE rez and a fantastic pet (oh, fluffy, you are worth your weight in gold). I am finding it a great match to DS with the resistance buff, heals (irregular, but better than a kick in the gonads) and debuffs.
Grab Mids and go to town - DS/DM gives you a very diverse array of choices for builds, depending on how you want to play your character.
Right now I have a level 26 Demon Summoning/Dark and I love playing with the toon. Once you can summon all your demons, it gets fun. I've read from different posts that demon summoning/pain is the way to go. I''m not sure which secondary power i should choose. Also I've been looking around for a decent build on demon summoning/dark but I just can't find the right one yet. If anyone has a build they would like to share, I would love to have a look at it.