Michiru and Kaoru becoming Cure Bright and Cure Windy was not a normal ability of theirs, but a one-time move made possible by their bonds with Saki and Mai.Thus why they've been absent from later teamups.
Michiru and Kaoru were originally humans.When the other members of Dark Fall died, they returned to their original forms — even Moerumba, whose original form was heat, was able to shape glass into his image before finally expiring to show what he'd been. However, the twins stayed the same every time they died or nearly died, except when the spirits revived them (twice) in the ending arc, when their eyes stopped looking creepy and they started to look more human. Could that have been their original form? If so, it would explain why Akudaikaan and Gouyaan didn't just kill them when they rebelled, but instead sank them to the bottom of the ocean. If they were to return to their original form as humans who didn't need the power of destruction and ran on a normal life force, that would have been spectacularly counterproductive.