Honestly I think Rosa's problem is that she's just very, very bland. Like, imagine the most generic perfect girlfriend princess in a fantasy RPG and that's basically Rosa. Like you know how total idiots think that Aerith is this pure perfect demure girl who will ride Cloud's dick all the way to Male Fantasytown and not a character with some traits of her own? That's Rosa.
In the grand scheme of awful characters in this series, I think Rosa's biggest crime is just being unflavored.
Nope, her stats suck ass too, especially when you try to naturally max her out and you notice that she just gets a +1 growth in everything while everyone else excels post Level 70. Rosa's only virtue of being in the final 5 is being Cecil and Kain's love interest and being the only good white mage there because Cecil sucks at studying and Rydia "forgot" White Magic. That's really her only claim to fame. Especially in the After Years compared to Porom. Lower Spir and MP, but she's a better attacker? Why would I use her as an attacker? Other people can equip Bows, so... *shrug* Even Porom can learn Dualcast in TAY and Rosa can't.
The only way to stat max her actually decently in via a few playthroughs in the DS version. And even in that version, she is always the most difficult character to stat max because of similar issues. Outside of that, if the party is malleable in end-game, I just take her out. Porom has a higher Spir than she does at the cost of just one Spd at even just Lv 70.
But hey man, Cecil leaves Rosa behind cuz she's sick in the interlude because he ain't got time for that.
At least Aeris is useful because she was basically planned as a Geomancer and that's what she does with her limit breaks and she was a more useful white mage based on a raw magic stat. Though to be fair, stats are completely negligible in FF7 anyway (especially since it's easy and E. Skill can break the game).
It isn't just that Rosa's a shitty character. Her stats are just
weird.
At least Vanille was a great debuffer and black mage, and Lightning was all-around balanced (and even then, I didn't even use her and stuck to a party of Fang/Vanille/Hope until very last post-game when Hope's vitality deficiencies were a bit of a hindrance).