Should Brainstorming Have Constraints?

Yes and no.  On the yes side, people feel demoralized seeing their "blue sky" ideas smacked down by non-transparent reality considerations and constraints articulated after people get fairly excited and attached to their ideas. So making known constraints transparent, and even soliciting others, prevents this and gives ideas opportunities to shape their growth in the direction of reality.

On the no side, the most innovative ideas make constraints irrelevant. The U of Michigan is innovating translucent solar power applications that in about 5 years will make every window in the world a potential source of usable energy. This makes panels and roofs and all respective constraints irrelevant. This kind of idea comes about when a group is also asked to generate options regardless of known constraints.

Groups need time to work on both sides of the creativity equation. That's where the magic happens.