I won't rehash all the amazing things DeCastro would bring to the O-Line. They've been well argued by better posters than I. One point has been only vaguely touched on, however; the consequences for the QB position.
With the addition of DeCastro it could be argued that we'd have an elite offensive line, capable of controlling the LOS. We'd posses a versatile “Speed & Bleed”tm running game as well as a “Strong & Long”tm receiver corp capable of taking a quick, short pass for long yards. Essentially, we would posses the perfect environment for a QB to succeed.
Assuming all the pieces fit as expected, an average QB would look like a Pro-Bowler. Many people are complaining about the lack of competition at the QB position going into training camp, but I see a pretty even field where any of them could rise to take the reins. The offense has been reconstructed to give Cassell every opportunity to succeed, but if he's not up to the task we still have an offense that would be perfect for a quality rookie or veteran QB.
A rookie wouldn't be asked to win the game. He would have a solid RB duo capable of gaining chunks of yards behind a solid line. He wouldn't be running for his life when asked to pass the ball, allowing him to stay poised and deliver the ball.
A starter-quality veteran would be able to wring the greatest performance from those around him. He'd never have to get dirty. Need three yards? “Hillis, you're up!” Power-O. Defense stacking the box? “Moeaki, fetch!” Got a short CB covering your guys? “Bowe and Baldwin, jump ball!” Got them off balance and need the KO? “Charles, do your voodoo!” A quality veteran would be drooling over the tools at his disposal.
Getting DeCastro takes an O-Line from good to Elite, and creates the perfect environment for a solid QB to not only succeed but to EXCEL.
If Cassell is that man, fine. If he's not, then his replacement deserves the best environment to succeed as well.