The Jagen is a character that fulfills the role of a mentor figure for the Main Character (most of the time). As such it’s realistic that this character is usually the strongest unit you have at the beginning of the game but grows slower and has lower Growth Rates than the rest. The perfect Jagen Character for me is a unit that is very useful for the early game and stays a viable option after that point. However he noticeable starts to fall of and will inevitable be surpassed if not replaced by other units after they promoted etc. It’s both a powerful tool to the player but also a test if he trains up only him or others. Kinda like “If you rely too much on others” you will get in trouble if you only use him… Except that in most games you actually get away with training your Jagen or only one unit for the entire game as long as you know what you’re doing. But personally I have more fun to train up a whole army of good units.
In my first FE7 Run ever I actually used Marcus. I loved to see him being so strong and a solution for a dense situation. His crit animation was awesome. And while he was very strong he wasn’t immortal. I did notice though that he grew slower than other units which was logical. He clearly must’ve been an evolved version of Kent and Sain. I didn’t fed all the kills to him because it was logical to me that he would eventually grow less good or at least much slower. But sadly this run only went on to the first Dragon Gate mission. I remember I promoted Kent but never touched that save file again. I actually can’t continue that run anymore because I deleted the save file years ago.
Afterwards I was following this good advice to act like Marcus isn’t there. To not give him kills and waste Exp. Now the thing is that you can decide to not use him of course but thinking back how that advice influenced me it’s likely that others followed that example as well. Which is kinda sad. Marcus and other Jagens are everything but Exp Thieves. And it’s sad that a majority of players only sees them as those and even newer ones Post Fates will probably never know what such a character is.
Personally I use them nowadays. They’re strong units that either fall off or behind the cavaliers you train up. But the thing is just because a unit will eventually surpass a strong unit doesn’t automatically makes that unit to something like Base Marty or anything. I think it’s cool that the normal difficulties don’t require such a characters but give you the option to make your life easier (and show you how other units like this could look in the future) while on harder difficulties you really learn to appreciate them. They are also not lame invincible units like I said previously. I mean it’s probably due to my stupidity but I managed to get Jagen killed in the first chapter of Shadow Dragon… twice. One time on normal.
Also another option you have is that once you don’t need your Jagen anymore that you can simple switch that character out for another one. Using a unit once don’t put you on a contract to always take it with you.
Using Jagens or not depending on the reason can tell a lot about a player. And one of the reasons to not use him is a thing that I’d really like to disappear from the mindset of new players or those who give this as the first advice. But this is only about this particular reason. If you decide to not use Jagens because you don’t want to have a strong unit at the start or you simple enjoy training up the units with more potential it’s more than fine.
About Jeralt in Three Houses - That game generally needs more units. Or no… It would need to make existing characters with voice lines, artworks, models, etc. playable. There is no reason why Jeralt, Judith, Nader, Ladislava, and so on we’re left behind other than “We didn’t have time to make support conversations” or some other crap like that. You did release a paid DLC character with everything but this too so why not… Sorry but I jut don’t get why they were so stubborn to only focus on the students and just a hand full of other characters for this game. I don’t see a reason why to not make them playable especially since Judith would serve the same role as Gilbert in Azure Moon and later on Jeritza in Crimson Flowers. And to an extent Seteth in White Clouds although he is playable in the majority of routes. Jeralt still could’ve been killed off and you could’ve simple banned him from teaching like you did with Dimitri in Azure Moon Post Timeskip. The existence of Jeralt in the early game (especially with the option to bench him) would’ve make Maddening Early Game a lot faster (or at least easier). Also I don’t see a reason why Jeralt wouldn’t be controllable at least in the missions where he is an NPC. Maybe they we’re afraid that you would train him or invest too much in him. But by simple giving him an Exp-Penality, forbid him to learn other skills, make him inaccessible for some missions like the tournament at the start and the gondor field battle and so on the game would’ve made it clear that this unit is just someone that watches over you and is helping you.
You could argue that both Byleth and the lord (especially Edelgard and Dimitri who already had combat experience before Garreg Mach) kinda are in this role of a Jagen. They’re mentors for the rest of the class. And better than most of your students at the beginning. But still - I think Jeralt and other potential Mid-Game Prepromotes would’ve been a good addition to Three Houses. And maybe made it a bit more Permadeath-Friendly.
In the end I notice a trend that the developers focus more on units that start out very bad but grow to become strong units along the way. Beasts at the end if you want to say that. Which resulted in an entire game focused on that philosophy. It’s definitely cool and I see why many people enjoy it. For me Three Houses can be described as a journey with your students that overcome not only their lack of skill in combat but also their personal issues as they grow up - But since Byleth is a self insert or a very quite character that doesn’t act like a mentor figure at all (yet get’s all the praise since you’re in his shoes and have success) and even Jeralt being practically not there even when he is still alive, I feel something is missing. And let’s be real - Who cared about the fathers death in this game? He and Byleth had 0 interaction. Maybe a little bit but… Compare this to Greil.
It’s cool to have the option to use or to bench a Jagen Character. Taking away that option is bad imo. Especially since those units would be very helpful on the harder difficulties. And if properly balanced they wouldn’t be an easy win button. Jakob and Felicia proofed that they can also do Jagens that grow good but not perfect. Units that not nessecarily fit the mentor role but are always on the side of their lord and are viable from start to finish. If they just want to get rid of the potential punishment that could lead to bad feelings since you obviously made a bad descision and kept feeding kills to that units for multiple hours which results in you having trouble to continue the game - This is the template to follow. Or something along these lines. Just like with the fear that permadeath will screw you over the idea with a Jagen being an Exp-Thief is usually not something to fear in the majority of Fire Emblem games. Games that are designed properly around that give all your units more than enough Exp to grow. It’s usually nothing to worry that much about. And especially on normal mode even the more modern games are more than beatable. The disappearance of the Jagen to me is another design descision in modern FE that is similar to the descision of making a smaller cast and the game less permadeath friendly.