I don't think you can have it both ways mokler. What's the difference in SB getting too old and needing to be replaced and SB being too injury prone/crappy and needing to be replaced? Either way, you need a replacement and having a QB who has practiced and learned the system is far preferable to having a total rookie. Both of your examples use the same flawed (IMO) logic. Unless you've changed your mind and you think SB will be with us for a long time?

The Cheatriots are doing the same thing with Mallett (picked in your third round scenario). I understand they got some good offers for him in FA this year.
Your point about anyone we're likely to be drafting being only a 3rd rounder (I don't want to draft one until the 4th) is a much better one IMO. Here's my thinking on that. I don't want to just draft a QB because we need one, I want to draft a QB who the brass thinks has the capability to be good but isn't there now. Even if he doesn't eventually become good he can help the team until we
can get a franchise QB. Think Cassel. He came in and led a talent rich team (and I expect us to be a talent laden team soon) to a very good record. Once he went to a team with much less talent his flaws became apparent but in the meantime he held the fort and they got a good draft pick when they traded him. That's not going to happen with a Vick or Freeman type.
As for waiting for next year, again I go back to my original statement, we lose a year of development.