I understand that P2 is an inferior Chansey but what the hell can P2 do that Chansey does better? I mean, P2 can't Wishpass, SR, Cleric, and Chansey is bullshit at offense. They are two completely different things that just happen to have the same typing and use the same item to function.
They're both very bulky things that are usually used to switch into special attackers. Chansey has good physical defense, but you usually don't want to be switching her *into* a Band Flygon Outrage, say--both of them would rather be switching into something like a Scarf Flygon Outrage some of the time while switching into special attackers more often (though you can EV Porygon2 to be able to take Band Outrage quite well, opting out of the special defensive spread, which isn't a bad idea). Except for the extra things given by Trace, you can use both of them to wall a lot of the same stuff--difference is, of course, Chansey does it better, due to taking special hits about 46% better.
I bring up the comparison because currently Porygon2 is just as bulky as anything commonly considered bulky in UU. Cresselia edges it out only very slightly (by 4-5,000 on each side), and that is just absurdly good. Even Snorlax, who is a top choice for special wall, is only a tiny bit bulkier than P2 on the special side, and much less so on the physical side. Specially the difference is like 183k to 177k or about 3% better. For an actual damage comparison instead of a direct durability comparison:
252 +2 SpAtk Life Orb Mew Aura Sphere vs 252 HP/252 SpDef Snorlax (+SpDef) : 54.96% - 64.89%
252 +2 SpAtk Life Orb Mew Aura Sphere vs 252 HP/252 SpDef Eviolite Porygon2 (+SpDef) : 57.22% - 67.38%
So they're taking similar amounts of damage from special things. But as the durability calcs suggest, Chansey is taking much less. Something like:
252 +2 SpAtk Life Orb Mew Aura Sphere vs 252 HP/0 SpDef Eviolite Chansey: 38.92% - 45.74%
So prior to Chansey's descent (if it descends), Porygon2 is one of your best choices to switch into strong special attacks while also taking unboosted physical hits, if not your best choice due to its ability to also use Toxic/TWave, recover, and threaten your opponent with 105 base SAtk. But Chansey is pretty much superior to this guy at doing that, except with much more utility instead of offense.
Analytic P2 is a different beast, and it doesn't do quite what Chansey does. With what basically amounts to a free Life Orb boost, AP2 can 2hko a lot of things it otherwise could not. For instance offensive P2 is ordinarily doing this much to offensive Mew:
252 SpAtk Porygon2 (+SpAtk) Tri Attack vs 0 HP/0 SpDef Mew: 36.36% - 43.11%
But after Analytic:
252 SpAtk Life Orb Porygon2 (+SpAtk) Tri Attack vs 0 HP/0 SpDef Mew: 47.51% - 55.72%
If Chansey does come down, it will make P2 a sad pony. But P2 can still hit stuff very hard, so it's entirely possible to use it as a bulky offensive Pokemon, like a Specs Slowbro might be used. The Analytic boost goes a long way.