Sludge Bomb's chance to poison shouldn't be dismissed either, it's the same as Scald or Lava Plume's chance to burn.
...and give reason for Conkeldurr to switch in?
252 SpA Gengar Dazzling Gleam vs. 252 HP / 4 SpD Conkeldurr: 248-292 (59.9 - 70.5%) -- guaranteed 2HKO after Leftovers recovery
252 SpA Gengar Sludge Bomb vs. 252 HP / 4 SpD Conkeldurr: 208-246 (50.2 - 59.4%) -- 80.9% chance to 2HKO after Leftovers recovery
Guaranteed 2HKO compared to chancing it?
252 SpA Gengar Dazzling Gleam vs. 0 HP / 4 SpD Hydreigon: 384-452 (118.1 - 139%) -- guaranteed OHKO
252 SpA Gengar Focus Blast vs. 0 HP / 4 SpD Hydreigon: 284-336 (87.3 - 103.3%) -- 18.8% chance to OHKO
So, once again, guaranteed?
252 SpA Gengar Dazzling Gleam vs. 0 HP / 4 SpD Mienshao: 262-310 (96.6 - 114.3%) -- 81.3% chance to OHKO
252 SpA Gengar Sludge Bomb vs. 0 HP / 4 SpD Mienshao: 220-261 (81.1 - 96.3%) -- guaranteed 2HKO
Well...there...?
252 SpA Gengar Dazzling Gleam vs. 252 HP / 252+ SpD Sableye: 166-196 (54.6 - 64.4%) -- guaranteed 2HKO after Leftovers recovery
252 SpA Gengar Shadow Ball vs. 252 HP / 252+ SpD Sableye: 124-147 (40.7 - 48.3%) -- guaranteed 3HKO after Leftovers recovery
...zing?
252 SpA Gengar Dazzling Gleam vs. 4 HP / 0 SpD Zygarde: 182-216 (50.8 - 60.3%) -- 89.5% chance to 2HKO after Leftovers recovery
252 SpA Gengar Focus Blast vs. 4 HP / 0 SpD Zygarde: 137-162 (38.2 - 45.2%) -- guaranteed 3HKO after Leftovers recovery
Take that, hopefully-buffed-next-game Dragon!
252 SpA Gengar Dazzling Gleam vs. 36 HP / 0 SpD Haxorus: 236-278 (78.1 - 92%) -- guaranteed 2HKO
252 SpA Gengar Sludge Bomb vs. 36 HP / 0 SpD Haxorus: 198-234 (65.5 - 77.4%) -- guaranteed 2HKO
Umm...no difference?
I said this before, but I simply find Dazzling Gleam a surprise shot at Fight and Dragon-Types that think they're safe against Gengar. If one has everything covered, Dazzling Gleam on Gengar becomes somehow worth the while...guess it's more "in the eyes of the beholder".