First Jirachi is a mediocre Pokemon. I've been using it a ton and it just invites everything in; Gliscor, Garchomp, Scizor, Heatran, Skarmory, Feraligatr, Manaphy... It is one of the most mediocre Stealth Rock user in the tier because it loses to every hazard remover, even if Starmie doesn't enjoy U-turn that much. Jirachi is indeed able to come on most Latios sets (LO Earthquake is dealing quite a lot), however it is a liability for a lot of balance team to have such a Pokemon that invites in every offensive threat and is just a U-turn machine.
Then Heatran. Recover Latios doesn't care at all about Heatran, especially if it has Calm Mind. Also you probably never have switched your Heatran into Surf or Earthquake Latios, this Pokemon ain't kidding.
4 Atk Life Orb Latios Earthquake vs. 252 HP / 0 Def Heatran: 328-390 (84.9 - 101%) -- 87.5% chance to OHKO after Stealth Rock
252 SpA Choice Specs Latios Surf vs. 252 HP / 252+ SpD Heatran: 204-240 (52.8 - 62.1%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
I have no experience playing or facing Specially Defensive Mew, so I can't talk much about it. It may be an hidden gem that no one is using currently, but I would tend to say it is just a niche Pokemon at best.
Slowking is also getting owned by Recover variants of Latios due to Slowking being unable to do anything to Latios but statusing it with Thunder Wave / Yawn. While this can work, if Latios gets burned switching into your Scald or just switches out as you click Yawn, it doesn't work. Unless you run Nasty Plot or Calm Mind with Ice Beam (does anyone run that?), you're not going to threaten back Latios (0 SpA Slowking Ice Beam vs. 0 HP / 4 SpD Latios: 120-142 (39.8 - 47.1%) -- guaranteed 3HKO). While in return Latios does more than Regenerator heals with Thunderbolt and Draco Meteor, and can even setup on you with Calm Mind.
Those Pokemon are hard to justify even with Latios around or don't even handle it well! Finchinator is right in his analysis: if you run something defensive you have to run Blissey, Scizor, or Specially Defensive Clefable. If you use offense, you're running Ice Shard + a Steel + a revenge killer. You can think this is not so restrictive (what I disagree with), but no way you can say there are other options and mention craps like the one you mentioned.