Steve Young was drafted in the 1st round by Tampa Bay. Aaron Rodgers was drafted in the 1st round. Warren Moon wasn't drafted at all. Tom Brady was drafted in the 6th round. Case Keenum wasn't drafted (I keed, I keed!!!). Joe Montana was drafted in the 3rd round.QFT, but I'll play anyway.
First rounders have never sat. Oh, I'm sure someone will come up with an example, so whatever, but the trend has not been for first rounders to sit and develop. But teams used to have guys sitting that they'd develop behind the starter. Sometimes it worked out, sometimes it didn't, but there used to be a higher percentage of QBs drafted in later rounds that sat and developed for a couple years. They'd get some mop-up time or maybe start a few games if the starter went down, and then later in their career, they'd turn into decent starters, sometimes very good. Even the guys that started early often saw limited spot duty in their rookie year.
Circumstances vary, but I can think of Steve Young, Brad Johnson, Jeff Hostetler, Stan Humphries, Elvis Grbac, and Lynn Dickey. I'm sure if I looked far enough into it, I could come up with some more. A few of these guys were drafted really late, but some were drafted in the 3rd-ish rounds. Coaches worked with them in the hopes that they'd develop and some of them did. Now teams spend a ton on their starter and don't bother too much to develop a later round guy. It happens, but it's not as common as it used to be.
Maybe it wasn't exactly common back then, but as the game has increasingly put skirts on the QBs and the paychecks have risen at the position, teams put a much lower emphasis on ensuring there's a real viable option at backup. The Colts are the extreme example - hell, they had exactly no one behind Peyton Manning for years.
There doesn't seem to be a tried and true method of drafting a QB. For every Peyton Manning out there, there are 3 Leif Garret's or Jamarcus Russell's. Some QB's learn under fire better than others, some need the time to sit and learn how to be a pro. Our very own David Carr could well be a prime example of that. Who knows what could've come of his career if he'd been allowed to watch a seasoned vet go out and get slobberknockered behind a horrible OL for a couple of years?
IMHO, I'd rather see a draft pick, regardless of draft position sit for at least a year and learn how to prepare like a pro. Just MHO.