Well, I'd say they were a better choice for inspiration than it could have been. Anyhow, it doesn't matter - the tips mentioned here don't depend on whose signature it is.
Actually, that is an interesting philosophical question. I'd have say, the main reason to collect signatures is as a memento, to remind you of the experience. Also, it is a way of showing your support for those you respect. Finally, it is a method for showing others your pride and happiness at meeting whomever. I personally don't collect signatures, but it seems like a reasonable thing to do.
Finally, why Kamen and friends? Because mainly, he founded this competition that we enjoy so much, and because they have worked on things that help many people, which we admire.
-Sam
PS.
Thank you for a question that I would not have thought through had you not asked.