I believe Rayek said it himself they went into his body because they needed Direction. Even in death they still needed to be guided and instructed. not because they needed a physical nexus.
They needed directions to get to the Palace? Like MapQuest or something? :D Elves go there automatically after death. Unless they have a specific reason not to, plus a place elsewhere they can inhabit. The Gliders as a group had enough collective power to defy the call of the Palace for a short time. But they still needed a physical Rayek to focus that power so they didn't wind up as individual spirits hanging around the joint when they finally made it there. Winnowill directed their thoughts and powers while they still had bodies, but moments after they lost those bodies they anchor to another body! They didn't just hover around Rayek like the Ghost of Christmas past, they inhabited him and caused specific physical changes to him. Like Winnowill. Also like Winnowill he didn't seem to be conscious of the change until after someone pointed out to him.