Mow Rotom and Sylveon are both kind of common, so you should probably keep Sludge Wave to hit them.
Commented 1 Hour ago by sumwun.
This is definitely true. Sylveon and Mow Rotom had a 20% and a 12% usage rate, respectively. In addition, it gives coverage for both Dhelmise and Convey which both have a 7% usage rate. This provides good coverage, but this is actually not the move Smogon give as a 4th move. They give Snarl as a 4th move, which is boosted by Punk Rock and activates Throat Spray, which is nice. While Snarl doesn't cover many things outside of Bronzong, it still is useful as a Throat Spray activator for when Boomburst isn't usable. These are for greater than 1760 ELO.
Edit: I totally forgot Smogon.com/stats existed, so let's include those here. I am going based off of 1500-1630 ELO and will also mention 0-1500 ELO. In the first group, Sylveon had a 9% usage rate, Mow Rotom and Dhelmise had a 5% usage and Comfey had a 8% usage rate. In 0-1500, Sylveon had 9% usage, Comfey had a 8% usage, Dhelmise and Rotom-M had a 5% and 4% usage.
However, I still agree with sumwun that the ability to cover the 'mons previously mentioned before is better than an additional Throat Spray activation method with some additional coverage, so I say to keep Sludge Wave as a Poison STAB.