The problem with Poopy Blast is that you have a 51% chance of missing at least one of those turns. There's also the issue of running Modest Zam - I personally go for Timid, to outspeed RP Rhyperior, Ludicolo in the rain, tie with Sceptile, tie/beat other Zams, and beat Espeon - a fairly short list, admittedly, but enough to sway me (though I also use Rock Slide over Stone Edge with no exceptions, so whatever). In the case of Chansey, keeping Zam in is a very risky prospect - do you want to gamble another hit and a good roll to take it out? If you don't it'll stall Poopy Blast out of PP or Paralyze you, or even KO with Seismic Toss.
Alakazam is looking much less attractive now than it was because I've seen a ridiculous amount of priority recently, even more than usual. Take out Venusaur, there's Arcanine. Or Azumarill. Or Spiritomb. Or Houndoom. Or Technitop. Or even Donphan. And it seems like the whole tier can learn Sucker Punch, and you never know when it's going to strike next. Sub Zam can get around that to an extent, but Encore reduces it to 2 attacking moves and it's not easy keeping a 60-hp sub with 130 defense backing it alive.
Also I'm really tired. Please replace incoherent sections with passages from Stephen A. Douglas, cause dang that guy could talk.