So if Rotom-Wash can't take repeated Dark Pulses, then doesn't that show that there is little need for Hidden Power Grass? Few Rotom are Specially defensive nowadays.
For one, HP Grass hits 50% harder than Dark Pulse. Two, using Dark Pulse means you take neutral damage from both Rotom-W's STABs, while using HP Grass let's you resist them. Since Rotom-W will probably go for Volt Switch against Greninja, this is how much damage you will take if you go for HP Grass first:
4 SpA Rotom-W Volt Switch vs. 4 HP / 0 SpD Greninja: 52-62 (18.1 - 21.6%) -- possible 5HKO
Factor in Life Orb, and Greninja takes 31.6% damage max. Meanwhile:
252 SpA Life Orb Greninja Hidden Power Grass vs. 252 HP / 0 SpD Rotom-W: 205-244 (67.4 - 80.2%) -- guaranteed 2HKO after Leftovers recovery
A minimum of 67.4% against Rotom-W with HP Grass, where Dark Pulse fails to 2HKO without SR, and even then Rotom-W has a chance to live two hits.
In a lead matchup between the two pokes it's clearly beneficial to have HP Grass, instead of having to use Dark Pulse or be forced out (generally the latter will be the case). However, having HP Grass also eliminates all need for predictions later on in the battle. Rotom-W can easily come in on Hydro Pump or Ice Beam and force you out, and even Dark Pulse won't 2HKO if you predict correctly assuming Stealth Rocks aren't up. If you have HP Grass, Rotom-W literally has nothing on Greninja barring the rare T-Wave. There's also the element of surprise, as Greninja don't often run HP Grass yet. Still don't see the benefits of HP Grass?