Martin
A monoid in the category of endofunctors
Not trying to invalidate any points, but if you read the second sentence of OO on the Whimsi analysis which is under construction atm you will see that it is there (second sentence means it's good when compared to the rest of OO, as it is mostly listed in order of viability). SubSeed isn't exactly a great option, but to dismiss it is a little rash considering that Whimsi is argubly the second best SubSeeder in standard (only behind Breloom).I quoted that portion because I happened to notice it and thought it was funny. But as a side note, the XY dex doesn't even mention SubSeed as an option. In general, SubSeed has been considered an unviable strategy on any Pokemon for the past two generations. It requires half your moveset and is very easy to counter. Even Whimsicott, who you think would be the best user, benefits more from all its other sets. The only reason it kind of works here is because it can sleep something first in order to not get annihilated while setting seeds, but there are still way too many answers to it.
I realize I just got through saying "we should not judge viability based on standard," but nothing about SubSeed itself has changed other than Leech Seed rising to perfect accuracy.
http://www.smogon.com/forums/threads/whimsicott-qc-2-3-written-up.3541088/#post-6301611
Anyway, IMO the best way to use Whimsi in this meta is to go with a pivot set similar to the one listed in the analysis which I happened to link above. U-Turn+Grass Whistle / Encore+Moonblast / Giga Drain / Encore+Stun Spore / Encore is more consistent for Whimsi than SubSeeding. Before the inevitable chorus of "why is Grass Whistle only a slash" I'm going to say RN that Grass Whistle is far from mandatory on Whimsi. Generally speaking, you should be using Encore much more than you should use Grass Whistle. The reason I say this is because the utility it provides is still the primary focus of its game, and you should only use Grass Whistle if Whimsi lures a major threat to your team (e.g. if your team is weak to Tran). The key issue with it gaining Grass Whistle to use is that it generates a minor case of 4mss and forces you to think much more about your team's needs. If your team struggles with setup, Encore should be used. If your team struggles with fast attackers, Stun Spore should be used. If your team struggles v.s. Sableye etc. then Moonblast should be used. [insert Water/Ground/Rock-type here]? Giga Drain. I think that Whimsi has been viewed rather two-dimensionally recently, with a huge level of hype surrounding SubSeed. Hell, I was pro ban until yesterday when I realised that SubSeed struggles relative to the meta. If you are using Whimsi, you need to think about more than "lets go with the grass whistle and the substitute and the leech seed and the blahblahblah" because that is a two-dimensional, predictable mindset that can be easily beaten. Lum Crobat is a pretty good answer to it on VoltTurn, and there are plenty of other ways to beat it pretty easily (a favorite of mine on tailwind is Lum Aurorus).
Speaking of Tailwind, it is actually pretty good in the meta ATM. With lots of good users like Whimsi getting buffs (Grass Whistle+Encore+Tailwind+U-turn/Memento is a fun set). Not much else to say than try it cause its fun AF.