It's not a bad plan. Trade a low round pick for a veteran QB and draft another QB to groom for the future. The Raiders still have over $20M of cap space after paying Schaub, meaning they can go after Alex Mack and possibly trade for Desean.
Here's a list of the Free Agent QB's, I'd probably rather have Schaub over 85% of the names that are on the list. If it doesn't work out, his contract has no guaranteed money and it won't count against the cap if he's waived.
1. Michael Vick (Jets)
2. Josh McCown (Signed two-year, $10M deal with TB)
3. Matt Cassel (Re-signed two-year, $10M deal with MIN)
4. Josh Freeman
5. Shaun Hill
6. Ryan Fitzpatrick (Signed two-year, $7.5M deal with HOU)
7. Jason Campbell (Signed one-year deal with CIN)
8. Chad Henne (Re-signed two-year, $8M deal with JAX)
9. Tarvaris Jackson (Re-signed one-year, $1.25M deal with SEA)
10. Kevin Kolb
11. Brandon Weeden (Signed two-year deal with DAL)
12. Colt McCoy
13. Matt Flynn
14. Kellen Clemens (Signed two-year, $3M deal with SD)
15. Luke McCown
16. Derek Anderson (Re-signed one-year deal with CAR)
17. David Garrard
18. Dan Orlovsky
19. Rex Grossman
20. Curtis Painter (Re-signed one-year, $795K deal with NYG)
21. Charlie Whitehurst (Signed two-year, $4M deal with TEN)