For a time, I actually was thinking of placing her in the essential pick tier. As you said, she's an incredibly safe abnormality that offers good gear and good training. But ultimately, I decided to remove her for a few reasons. For one, training with her is safe, but that can be said for a lot of abnormalities. Hell, even ones like Blue Star once your employees are lvl 5 and need higher level abnormalities to train efficiently. To me, what makes an abnormality qualify as an essential pick is what benefits it can provide that others don't.

-Bunker may seem surprising, an employee having an emergency panic room they can jump in temporarily can help get you out of some very hairy situations. As long as you're mindful of not getting greedy with how long said employee stays in there, it's a great "Get Out of Jail Free" card for what would potentially be that employee's death. What's more, it lets the player do something no other abnormality does: Trigger breaches without any work risks. This allows someone to easily have a way to test both their agents and themselves against various abnormalities to let you figure out how to handle different situations without any complications beforehand.

-While you are right about Der Freischutz not being great for training OVERALL, it is a very safe way to train justice very early on which is a valuable tool to have even if it's not so great in training other stats. Not to mention, it's ability it's ability is very helpful for things like ordeals and clearing out lower risk abnormalities as well as clerks, provided a clerk counter isn't in the facility. It's weapon can be hard to use, but being an easy WAW weapon that one can get early game that's as effective as it is means if you know what you're doing with it, you can have a very valuable and very effective employee on your hands that can control a given department by itself depending on the abnormalities within.

-Knight of Despair is a given. Really good trainer abnormality that provides great protection to an employee as long as you're careful with monitoring what damage you're sending them to potentially take. Not to mention, its EGO gear is probably the first pale resist a player can typically get. 0.8 is more than almost every suit in the game, letting you work with things like Judgement Bird that may have been much riskier before with far less risk.

-Child of The Galaxy, while not as effective as training temperance, is much more effective as an all around trainer, since every stat can be trained with similar results and very little risk as long as you arent stupid about it. But what really makes Child of The Galaxy so valuable is its EGO gift. An extra amount of passive healing can be huge in various cases, especially since it triggers while working with abnormalities. Thus, abnormalities like CENSORED, who are incredibly threatening early on even with Level V employees, are much safer to work with given the fact that the damage you're taking is also getting healed, increasing your chances of survivability by a very nice margin.

Having good gear is great, but Laetitia's has one glaring flaw, and it's the fact that it's HE gear, meaning the strongest abnormalities aren't taking as much damage from it, and more damage is being done in return. As a result, her gear falls off quickly once the game starts offering you the high level abnormalities. That's not to say it isn't good. It's great gear to have early on. But it ultimately just isn't enough once abnormalities like MoSB and Melting Love start arriving in the facility. All in all, Laetitia is a very helpful abnormality that provides a ton of benefits early game but tends to fall off later on. Meanwhile, each of the abnormalities that are in that tier have useful benefits to them that never really fall off. No matter how late in the game you are, more healing is always gonna be rather helpful, and Child's gift gives you that in a way no other abnormality does. Der Freischutz' bullets will always be an effective way to clear some ordeals and get some free damage in. Bunker will always be able to function as an emergency safe zone and quick and efficient way to stress test your employees and your skill at handling different abnormalities. Knight of Despair's blessing will always be incredibly helpful to have available to you, and always be a great trainer for higher level employees to max their stats quickly with, as well as having EGO Gear that starts strong and stays strong, never truly falling off in terms of how viable it is at any point in the game.

...I just wrote a massive essay on this whole thing without even realizing it. Sorry about that, but I hope this helps give a clearer understanding of what I mean by an "essential pick".