And if that TTar attacked with Fire Blast? You just lost your Excadrill. Or your Whimsicott. Either way you are now down 1 Pokemon and the match just started.Wrong. T-tar gets up rocks, but then instead of facing down, say, an excadrill, it's up against a subbed whimsicott. Cue the hard counter, whimsi is gone again.
And many celebis DO run hp fire, including the NP set, which is the best set around at the moment. Besides, your strategy required you to switch in 2 pokemon, so you just lost 12% on both w/rocks, while he didn't. If you u-turned at least you do some damage and can bring in something to threaten t-tar. Besides, who sais you have either of those pokemon? And what if you mispredict? Maybe you bring it in on a rotom-wash switchin and suddenly heatran has to run to something. If you say whismsicott it won't enjoy taking a hydro pump, then a volt switch.
What counter? Like I said name a Pokemon that hard counters Whimsicott, in the top 20, even 30, besides Reuniclus. There isn't any. If you are behind a Sub you have complete control over what happens next. If they lack a Grass type or a magical pokemon you can go to town Subseeding everything. You seed the TTar that is out and Sub seed it to death. If they switch you gain Leech Seed on their pokemon that is out now. Then you can Paralyze whatever it is to make the pain really begin. If Ferro comes out Taunt it then switch to your steel killer (Every decent team has one in this Meta) if nothing else Whimsicott is only 3HKO'd by Gyro Ball, and power whip fails to even break your Sub. U-Turn will always be an inferior option on a pokemon that has a priority Sub to scout.
BTW I am basing all of this off of the set of moves I run Sub/Leech Seed/Taunt/Stun Spore.
How the heck does Gliscor do ANYTHING to Whimsicott? U-Turn is the only option it has against Whimsicott. Priority Taunt makes it basically dead weight. Also even including the usage of Ferrothorn you will only be facing Grass pokemon ~12% of the time. Also, none of those enjoy being Paralyzed.Finally what do you mean when saying that grass types aren't common ?
In which metagame where you playing?
Ferrothorn is in every 4th team...There are also Virizion,Reuniclus,Gliscor,Breloom,Celebi,Espeon and some more that i may forget,which can shut down every attempt for subseeding.There is also Scizor which can simply u-turn out to get rid of leech seed and break your sub at the same time!!!Scizor,Ferro and Gliscor are in the top 10 of usage!Hoping that you don't see any of these complete counters to Whimsi in a team means that you are clearly hoping for a miracle...
So yes subseeding is very unviable and very situational and not worth over the momentum that U-turn gives you...