The Detroit Lions need a starting QB badly. Cassel gives you a proven QB.
Your other option is to draft Stafford at #1 and pray that he doesn't become a high priced bust.
The other factor is that if you take Stafford #1, you're not gonna get one of the premier OTs at pick #20. This means no Jason Smith, Andre Smith, Eugene Monroe, etc.
Scenario 1:
#1 pick for Jason Smith
#20 pick for Matt Cassel
Result: You've got the best LT in the draft and a proven NFL QB. Watch Cassel throw for 4,000 yards to Calvin Johnson and enjoy your first winning season in decades.
Scenario 2:
#1 pick for Stafford
#20 pick for fill in the blank prospect
Result: You have a QB of the future with high upside but who could also bust. Meanwhile you don't have an elite passblocker protecting his blindside. Watch the kid get sacked a ton and sit on your sofa and enjoy another losing season.
Scenario 3:
#1 pick for Jason Smith
#20 pick for fill in the blank prospect
Result: Your passblocking improves dramatically due to the elite tackle you've drafted but you've still got no credible QBs to throw the ball to Calvin Johnson. Watch an unimpressive offense and enjoy yet another losing season.
What choice would you choose? Do you have any other better scenarios than Jason Smith + Matt Cassel??