I've moved the spell priorities around in simcraft to where VT, and SW: Pain were above DP3, and it showed a DPS loss. test it for yourselves.
1 DPx3 Did 178.1k damage (FULL SPELL, DIRECT DMG AND DOT)
1 Mind bender did 163.8k damage (FULL MINDBENDER, NOT 1 SWING)
1 SW: Pain did 70.1k damage (FULL DOT NOT TICKS)
1 VT did 65.9K damage (FULL DOT NOT TICKS)
1 Mind flay did 42.1k damage (FULL CHANNEL, NO CLIP, NOT TICKS)
1 shadowy app did 25k damage
1 halo did 72k damage. (1 HIT, NOT MULTIPLE)
So when you use that 1 GCD of DP over refreshing a VT or SW: Pain, you hardly lose any damage, but you gain the massive direct damage from DP, plus the dot damage of it, then right after that GCD you get to put sw: pain back up, then VT. Also, Getting DP up first does more damage than mind bender. AND it does more damage than halo.
In theory, the logic behind getting DP up faster is with procs over the entire duration of the fight, you could get an addition, or multiple additional DP's up over than if you waited. More DP's, More damage overall. It's just like mind flay clipping where you are trying to get more ticks of MF.