I'm going to use an argument that I don't like very much, but many use it, Sanji's role in the crew is that of a cook, right? But he's still the third best fighter. Luffy (Captain and MC) and Zoro (Crew's fighter) are the two best fighters of course they will have more prominence in battle roles, not leaving Sanji to one side, but it's obvious that this mutation was something that was triggered during the second use of the raid suit (stated in manga), making it impossible to have a training "arc" for Sanji to awaken the exoskeleton, like Luffy and Zoro had respectively trained. Just my point of view...
I don't have any qualms with the nature of the powerup. My issue is with it being dropped in like Chopper's MP upgrade (or like the shit that happened in Enies Lobby) and not organically written into the arc.