Oh, thank you for answering this. I was racking my brain trying to figure out a way to explain why defensive cores aren't good in Doubles. Sometimes it's hard for me to transition my thoughts into words :P
I just want to make one connection with Point #2 and the italicized P.S.: Leech Seed stalling can actually work in Doubles/VGC, although pretty much exclusive to Ferrothorn and Mega Venusaur. This is because they are both bulky as heck and can dish out damage as well, meaning that they can afford to run pseudo-Stall. A Venusaur or Ferrothorn on the field, even alone, with double Leech Seed and no checks left on your side can be a real bulldozer.
Here's my two bits about the whole thing, which I'm not gonna write as an answer.
In Doubles/VGC, there is such a thing as a defensive switch-in. But it's different than Singles. I'll use the example of an Escavalier / Heatran / Jellicent Trick Room core. In Doubles, you want to have a resist to switch in to obvious SE attacks. However, you also want that switch-in to be able to BEAT the Pokémon firing off attacks.
In the core I listed above, we have Jellicent to set up Trick Room and Heatran and Escavalier to sweep. Jellicent hates being Spored by Amoonguss, and Amoonguss in general. So, switch in either Escavalier to take a Spore or Heatran to take a Giga Drain. However, the reason this type of defensive switching works is because both Esca and Heatran can BEAT Amoonguss, meaning that Jellicent won't have to deal with it later. If I switched in, say, Safety Goggles Swampert, I might be able to switch into a Spore, but all defensive value is lost to a consecutive Giga Drain because Swampert has no way to reliably check Amoonguss.
Likewise, Escavalier despise Fire attacks coming it's way. Hence why Heatran is on the team: to take Fire attacks via Flash Fire. However, it can also consistently DEFEAT most Fire-types with Earth Power, thus making it work as part of a defensive core. Jellicent can also remove Fire-types. Now lets' look at Jellicent's weaknesses. Dark-types are both resisted and hecked big-time by Megahorn. The aforementioned Grass-types are checked by both partners. Electric-types are handled by Earth Power and Drill Run.
Defensive cores aren't perfect. For example, Jellicent in immune to Heatran's Fighting weakness. However, the only Fighting Pokémon it can really check is Terrakion. For this reason, a Pokémon like Sylveon is added because it works in Trick Room and resists and checks Fighting-types. Her weaknesses are covered by Heatran's Fire STABs and Earth Power, as well as Esca's Drill Run.
This is what makes Doubles different than Singles. One Pokémon can't take on the world. Even three Pokémon can't do it perfectly. You have to have a team of 6-4 Pokémon that mesh together, cover each other, and can adapt to many environments.
TL;DR Defensive cores in VGC/Doubles have to have offensive capabilities as well. Pokémon like Escavalier are bulky, powerful, have a lot of resistances, and bring something to the team that supports it (Spore immunity with Overcoat). Pokémon like Blissey and Skarmory just… sit there and absorb a shot or two.