GS-Zubat is a smart man, though I wouldn't say SpD Skarm "laughs" at Manaphy. Surf is a 3HKO from any offensive set with neither rain nor Tail Glow boosts
WJC, pseudohazing obviously still has many uses, not the least of which is pushing excessive entry hazard damage onto the enemy team regardless of the pokémon you're facing. It scouts teams
(and avoids the embarrassment of your Skarmory getting trapped by Magnezone second turn), it gets rid of dangerous pokémon until they become more manageable later, and so on. But with a Leech Seed on you, you've already taken 25% damage just to switch in and roar/WW, not including SR or any damage from the opponent's actual attack. And with Leech Seed making it almost impossible to wall much of anything, the pseudohazer will likely have to switch next turn in a weakened state and be unable to stop the SubSeeder next time it sets up.
That's the difference between pseudohazing a SubSeeder and a stat-booster.
As far as the Latios comparison is concerned, Skymin is less susceptible to Pursuit, harder to revenge kill because it's faster, and more likely to beat its potential counters with "hax." Latios has its advantages over Skymin, great resists for one, but Skymin is more of a raw offensive threat. Latios is more hit-and-run, not being weak to SR and having to switch out of Draco Meteor debuffs.
"SubSeed hardly kills stall. ... Offensive teams are often the ones with no good answer to SubSeed."
More generally, stall relies largely on status and entry hazard damage to defeat its opponents. They don't get that damage if the opponent digs in for 10 turns with a SubSeeder, can't status a Substitute
(and Skymin is immune to TSpikes), and it typically has less offensive options for beating the strategy.
(Fewer priority attacks, faster pokémon, revenge kill options.) The only reason stall would have any success beating it is because stall teams are more likely to pack its most reliable counters.
Regarding the previously mentioned ST Tentacruel, I checked the damage and Air Slash is a 3HKO almost no matter what. :( Only a 43% counter,
I'm not sure how Blissey wins with Wish/Protect support
Wish/Protect Blissey "wins" simply by PP stalling Skymin. It can't push through enough damage to kill Blissey and Wish allows it to switch out of Leech Seed and defense drops without Skymin being able to kill the switch-in. Jirachi does the same thing better -- it can use U-Turn instead of a normal switch and is less vulnerable to Pursuit -- but cannot handle absorbing status at the same time, as BRN damage cuts into its HP too much and PAR only makes it more likely for Skymin to eventually flinchhax it down.
(Blissey has Natural Cure.) Calm Zapdos has solo viability with Pressure + Roost but the SR weakness really hurts and it dies if it takes a SF SpD drop.
None of these, of course, actually manage to
kill Skymin. If you run out of things to switch in and take hits while dumping Leech Seed + SpD drops, Skymin will eventually beat all of them anyway. A SubSeeder already set up is very difficult to beat with any single pokémon.
First, Garchomp is every bit the monster that he used to be. I absolutely love teams that use a scarf latias/latios as their garchomp check. I get off that swords dance, survive with haban berry and proceed to sweep in the late game.
Too bad SDChomp gets OHKOed by
Specs Lati@s, even through Haban Berry.

With SR, LO Latios too.