Haunter: Tbf, a lack of discussion implies that people are agreed on it being fine. I think the usage stats pretty clearly broke any arguments regarding those, anyways.
(Also, Blaziken was mentioned in the OP, so technically it's legit for this thread, right? ;o)
I'm sorry, but people who make those counter-arguments are acting like idiots - You can level-up that Mewtwo outside of battle. You can change that Shaymin's moveset easily. Can you legitimately change Blaze Blaziken's ability? No. Blaze Blaziken is its own Pokémon; while physical Shaymin is nothing but a gimmick that can be changed easily without altering the Pokémon - same for the Mewtwo.
Yo, my Ho-oh has 0 in every IV, is it allowed in OU now? :)
As a point, games are being played on POL.
Everything can be changed easily, so those grounds for a ban are just bad.
As for Deoxys, yeah, it's really easily shut down by a Taunter or an effective attacker. Most of the time, I just see it as setup-bait.
Latios is a little more difficult, since it's a lot harder to effectively take down, but I'm inclined to say it's not over-centralising and, as such, shouldn't be banned.