It's a funny concept, drafting based on "Best Player Available" rather than need. Every team says that's what they do, but the reality is, very few teams actually practice it. Denver didn't NEED a QB, but he was the best player available, so they got him. I suspect, 3 years from now, they'll really benefit from their foresight.
A good comparison, I think, is Joe Montana/Steve Young. Granted, neither Plummer nor Cutler even deserves to be compared to them on a talent or success basis, but I think the situation is pretty similar. San Francisco didn't NEED another QB, but they still went out and got Steve Young when they had the chance, because they absolutely loved his talent. The result was a much smoother transition from one QB to the next than normally happens.