It's why every IT department should have one or two openings permanently available in case someone that's really valuable comes along. Identifying that person can be hard, and to a large extent a hiring manager needs to trust the recommendations of his staff or friends at other companies, but when that great talent becomes available, you want to be able to secure it.
Sadly, as I've found from personal experience, this can be a dual-edged sword. I'm one of those "catches", yet a year later my company still doesn't know what to do with me. In the interim, I'm stuck in a position that utilizes only a very small part of my strengths. Frustration all around.
Yes, keep a slot open. But no fair catching a cat then expecting it not to yowl terribly while being shut in a wet cardboard box all day.