As an idea evolves, it will change shape, like the course of an airplane making its way across the globe. This idea can be left alone over time or constantly re-adjusted to stay on a healthy course (if your idea is traveling 600mph, you'll want to adjust frequently).
In the case of an idea, product, or process, if your enterprise invests only in validating the idea, it's like pointing an airplane at San Francisco and hoping for the best. When it comes to airplanes, you have navigation systems, wide panels of lights, and dozens of people ready to let you know when you've drifted off-course.
This is where we escape the metaphor. Innovation doesn't have a destination, but it does have a course. What keeps us on the course are those with the insight and experience to show us where we're drifting.
Open innovation is about creating and testing a perspective that has the advantage of intellectual diversity.