In every even-numbered continuity of Precure (counting both Futari wa Pretty Cure seasons and both Yes! Pretty Cure 5 seasons as one continuity each), there has been a HeelFace Turn for at least one female character who either became a Cure herself, befriended the Precures so closely, or fought alongside the Precures as a Cure. The emphasis on "even" is fitting considering that those characters are the ones on the most even terms with the Precures:
KiraKira★Pretty Cure à la Mode - Bibury does not become a Cure, though she is befriended by Ciel and later works at KIRAKIRA Patisserie in the episode after the one where is purified.
The fairies always choose teenagers to save the world instead of adults because they want to recruit warriors who are as fit and healthy as possible.
Additionally, because the Cures conceal their identities, missing school to fight would have far fewer repercussions than missing work. A teenager would, at worst, get lectured; an adult could get fired. Maybe part of why the fairies choose teenagers is because a teen's life is less likely to be ruined by a villain striking at the wrong time; an adult fairy would be likely to know this, while a younger fairy would simply prefer the company of fighters closer to their age.
Every time the Monster of the Week is defeated, all of the damage they caused disappear. The Pretty Cure are literally curing the world.
A recurring event in many Pretty Cure seasons... is the end of the world. Barren lands, people disappearing and even the destruction of the planet. Thankfully, everything gets better, but it makes you wonder what happens to anyone who isn't a Pretty Cure during those sequences.