I didn't know attacking types could become redundant. Anyways, Quagsire and Gastrodon wall Rotom-W 100% without HP Grass. So it's not that worthless to get past. If they're using something specific like Gastrodon or Quagsire Rotom-W could mess up their team which you'd definitely want to get past them so you could proceed to do that. The useful things you'd Will-O-Wisp usually get wrecked by rain boosted Hydro Pump. Dragonite is the best I can come up with who you don't want to be playing around with 75% accuracy on. Cradily would also be useful to burn but HP Ice wouldn't even do enough to risk a possibly +1 Giga Drain let alone trying to burn it.
If you're not on a rain team I agree Will-O-Wisp is the way to go but on one I think HP Grass is the best choice.
If Rotom-W's on a rain team, it'll almost certainly have Ferrothorn on it too. Since Ferrothorn can get in on Gastrodon for free, why bother running a Grass type attack on Rotom-W when your team already covers its targets perfectly well. Plus, if you don't want to use Will o Wisp then just run Substitute, it blocks Toxic from Gastrodon and prevents it from doing ANYTHING, whilst you can Pain Split and burn Substitute PP until it switches. It's also actually useful in general to block status and fuel Pain Split against opponents like Blissey.
On Rotom-W, Substitute > HP Grass. Also worth noting, Hidden Power Grass isn't mentioned at all on the official analysis, despite other Hidden Powers like Fire and Ice being discussed at length.