If you're Google, waiting until you absolutely know you need a specific talent seems like an approach that would definately keep fresh innovative ideas and creativity out of your company.
Imagine a young Mark Zuckerberg going to Google for a job. Zuck says "I got this great idea for social networking for college students." Google HR says "We don't have a position open for that right now. Plus you don't have a good resume and you're not a fancy-pants PhD. Don't call us. We'll get back to you." Zuck goes down the street, continues to work on Facebook, and then promptly hires away all of Google's employees. The rest is history.
What I want to hear from Google right now is "Patrick, we don't have a specific job for you yet, but we would rather hire you now anyway to prevent you from accepting an offer elsewhere." But that'd just be too damn easy, I guess.