Kate Rutter of Adaptive Path has a great presentation on the problem solving power of stickies. She describes how to use the baked-in simplicity of sticky notes to quickly work ideas through groups. While she refers to physical sticky notes, I think the simplicity maps over to the virtual sticky notes of Protonotes.

Discussions around prototypes should be an open, unstructured dialogue. Just as a prototype should be able to be changed quickly, the conversation around it should be freeform. The informality of a sticky note in direct context of a prototype is a great combo.