Calling him the 2nd best healer doesn't mean much when he's the only other healer besides Yukari (and I guess the MC if the need arose, and I hesitate to call Akihiko a "healer" with his grand total of two single target healing spells). And even that doesn't mean much because on upper levels, not a lot of bosses or enemies really targeted Yukari's weaknesses (where Ken's mudo weakness made him a major liabilty). Not to mention that Yukari gets all of her major healing spells 4-8 levels ahead of Ken in a game where even 4 levels could mean the difference between fighting a tough boss and being halfway on the entire next block of Tartarus. This is even further exacerbated due to the absurdly slow exp gain when the game decides you are overleveled at a certain block (where you can't just exp grind Ken to the occasion).
Actually, everyone short of Junpei and Koro can heal, but you're right in saying that there are 2 viable healers (3, but the MC is primarily a damage dealer), but that's exactly my point; Ken has a niche.
Before I address why Koro sucks, I'd like to point out that it's incredible disingenuous to state that Yukari's weakness is less prominent than Ken's. Out of the 12 major shadow bosses, 2 of them use primarily Zio skills (Hierophant, Hermit)
and none of them use Mudo skills. One of these bosses exclusively uses Zio skills and is arguably one of the hardest bosses in the game. The only bosses that use Mudo skills are Jin and Chidori (the former also uses Hama spells) and the final boss (who uses everything.) You're right in stating that gains Mediarama significantly earlier than Ken, but her being better than him is not in dispute. My argument is that there's a point to using him over Yukari, such as:
-More coverage. Yukari is a sitting duck against enemies that resist Wind. Ken's access decent physical skills and Hama skills make him a viable choice in most battles.
-His natural weakness aren't as bad. Ma-zio spells are significantly more common than Ma-mudo spells, and Mamudo skills can be covered by homonculus.
-He has better synergy with Aigis. Aigis is the best boss killer in the game, whereas Yukari is FORCED to use the Storm Bow to eliminate her Zio weakness.
-Gains access to wind coverage via weapon fusion
I don't really feel like getting into Ken vs Yukari debate (because it's been done to death) but Ken is definitely viable character, no question about it.
Koromaru is better because he is actually not a walking Once More turn to enemies in a lot of the higher level blocks (and quite the opposite) in a game where almost every party member has weaknesses that you can't do anything about (unless you're playing Persona 3 Portable and have the added benefit of guard and direct control). Plus, his offensive stats are not terrible and for the enemies that are weak to mudo, which often happen to be the most annoying, Koromaru can spend SP where the MC doesn't have to.
There is a ridiculous amount of selective bias and incorrect information in this post. OMT's can be mitigated with smart team setup/first turn advantage. His double immunity is nice, but nearly good enough to justify having him used over any other character. I don't see any strong reasoning in your argument for Koro to be recommended over using any of the other staples.
So here's the deal with Koro:
-Both of his offensive stats are objectively subpar
-He has an overkill agility stat
-All of his skills are magic based
-His equipment options are seriously limited.
He doesn't have a shoe slot.
-He has decent coverage with Fire and Mudo skills. Mamudoon is definitely cool, but unlike Ken he doesn't gain Mudo boost so they're inaccurate.
-He adds nothing to do team. MC can cover Fire & Mudo skills fairly easily (You get Alice/Die For Me! at level 49, so you're covered on the mudo front)
-He has low SP
-Shadows tend to be weaker to hama skills than mudo skills (but in fairness, they're more likely to use mudo skills)
So in conclusion, Koromaru is never really ever more than passable in P3. He's decent in random battles, but never a strong contender boss battles.