The reason I like high HP isn't for a single fight, it's the overall sustainability over multiple fights. The main reason I find defense less useful currently is that each fight in 12-5 (except for the final boss) only lasts 2-3 turns so I don't find the long term sustainability of defense to be helpful when since the enemy only gets in 1 or 2 rounds of attacks. With equipment, all of my girls have over 2k defense anyway, so the only real issue is the mosquitoes. Assuming mosquitoes do % based damage that isn't affected by defense, the only way to really mitigate them besides Toad Lilly and healing is more HP. Yes they'll do more damage, but a girl at 30% of 13k HP is still more health than a girl at 30% of 10k HP.

With high overall HP girls + Edelweiss's heal and heal pickups (which I'm assuming are also % health based), the main goal is to not have any of the girls die before they can get healed, and I've had the most success with that using high HP units. Also, because of the way it's designed, an Edelweiss squad with high HP girls can literally solo the entire map by healing up with nests between the fights, making the rest of my teams pointless except for picking stuff up and maybe finishing off the final boss if I get unlucky.

Maybe other people's personal experiences have been different, but in my experience I've found that high HP/low DEF units are a lot more survivable over multiple fights than the low HP/high DEF units. Maybe it's because of the ability to put defense equipment on high HP girls, but it seems like the increased defense never makes up for having less HP. I also think defense would be more relevant if fights actually lasted longer than a few turns (which is also the main problem I have with Lily Rubellum...her ATK buff is basically useless for most fights).

However, I do think it's stupid that "total power" is just their HP, ATK, and DEF added together. 1 point in HP is not the same as 1 point of ATK or DEF...