I was trying to keep my list to back-ups that had been traded. Yeah nearly every HoF QB was a back-up at some point in their career. Hell, Joe Montana held the clipboard for Steve DeBerg for the 1979 season...Dan Marino was a back-up for about half a season (to David Woodley if I remember right). Drew Brees was a back-up to Doug Flutie (but only for part of a season I think).
But to the detractors, yes, the odds of a trade for a back-up QB turning out highly successful are low. For every Brett Favre there are a good number of Scott Mitchell type results. But frankly those odds aren't significantly different than drafting a QB. Think about it...for every Petyon Manning there's a Ryan Leaf. For every late round Tom Brady there are a few Danny Wuerffels.
The fact is that selecting and developing a quality QB is VERY difficult, and too many coaches are simply incapable of helping in that development process. To many owners and GMs have a revolving door of coaches and systems giving a young QB no chance to develop.
Cassel is no different. I think he is capable of being decent, but he has been put in a similar position that Alex Smith was for years with new coaches and/or systems each year, little to no consistency with personnel & O-line, etc. I'm not saying he is the answer, but I see a lot of parallels between this situation and many others where the QB turns out to not be the problem (see the Niners and Alex Smith as an example). Heck when he left San Diego many people thought Brees was the problem with the offense under-performing. Guessing they'd like to re-think that move?!