WARNING: The following contains spoilers for My Hero Academia Chapter 349, "Battle Flame,” by Kohei Horikoshi, Caleb Cook and John Hunt, available now in English from Viz Media.
As My Hero Academia's current war rages on, the estranged son of Endeavor, and League of Villains member, Dabi continues to literally turn up the heat. His Quirk, Cremation, has always been a double-edged sword since its flames ate away at his body as well. The pain never stopped Dabi from using his Quirk as much as he could but one can't deny that it hindered him from reaching his fullest potential. In his current state, it's highly unlikely that Dabi could emerge victorious from his fated match with Todoroki Shoto, leaving one to wonder why All For One never took care of Dabi's greatest weakness.
Even as he burned, Dabi continued to produce a steady stream of flames that threatened to overwhelm the hero team assigned to him. Endeavor's sidekicks, Burnin, Kido, and Onima were barely able to withstand the heat while Tenya Iida could not even approach Dabi for fear of overheating his calf engines. It was only Shoto, who'd recently perfected control over both aspects of his Quirk who could bear to approach Dabi and even engage him in conversation.
In response to Dabi's taunt about Shoto playing the part of Endeavor's pawn, Shoto told his brother that he'd made the decision to confront Dabi on his own. Dabi concurred, musing that the war was a result of many individuals' suppressed emotions suddenly exploding. Shoto pressed on, inquiring why his brother refused to return home after surviving the blaze that was thought to have killed him. The question piqued Dabi's interest and he finally revealed his face as he promised to eventually answer Shoto's question.
Dabi's face reveal showed him looking worse than he ever has. The remaining strips of skim that clung to his face had been burned clean off, along with his lips and most of the skin on his jaw. The flames he was making were much more intense than any Shoto or Endeavor had produced and his face reflected the price he had to pay to make them. The Quirk's risk has been made apparent since Dabi debuted in My Hero Academia but its persisting until this point of the story must be a gross oversight on All For One's part.
Dabi was the firstborn son of Endeavor's Quirk marriage; a process meant to create the perfected form of Endeavor. Though Dabi inherited a stronger version of his father's flame Quirk, his body took after his mother's. Instead of gaining a fire-resistant body, he got one that was capable of withstanding icy temperatures. Even so, the limited use of his Quirk made him an invaluable League member. During the Training Camp Invasion, he provided cover for the secret mission to kidnap Bakugo and nearly burnt several heroes to crisps in the Paranormal Liberation War.
It seems natural that someone, as calculated as All For One, would grant one of his most valuable allies as much insurance as he could, and yet, Dabi continues to wither away with every new use of his Quirk. Whether by directly giving him a Quirk that countered his flames' effects on his body or having Dabi undergo a similar operation as Shigaraki did to boost his regeneration abilities, All For One had several options to deal with Dabi's in-built weakness once and for all. The failure to do so might be the reason the League's famed Firestarter might face an inevitable defeat at his younger brother's hands.