There would be no effective difference between Castle or the other guy. This guy wasn't even a RINO. He was a Democrat. Even if the seat in Delaware is lost, which isn't a given in this bizarre year, it was worth it to send a message to the GOP leadership. Look at it this way.
With the Republicans in control of the House, which is a pretty likely outcome, none of Obama's legislation or egregious budgets will get through.
That leaves confirmations and treaty ratifications for the Senate, but the Republicans are looking to cut down on the Democrat majority, and this will make it tough since they've barely been able to get anything accomplished even with a supermajority. Also, the chance of the Republicans taking the Senate are so remote, even with Delaware, that it's not worth considering. Everything would have to go right in every race.
By the way, I think it is likely the Democrats would have been able to bribe Castle into switching, or else he would have obtained important positions on committees from the Republicans to keep him nominally loyal. We are better off with either Coons or O'Donnell than with Castle.
I'm also not writing off O'Donnell in this race, and plan to support her in every way I can. This is very weird year and she has a better chance to win than many would believe.
As far as the nonsense about the GOP being able to advance an agenda goes, they don't have one to advance yet (we will see what they come up with after the midterms), and if they did, it won't matter much whether they control the Senate or have rough parity in the Senate.
All of this also has no impact on Obama's re-election. That depends largely on what the economy is doing two years from now and who the Republicans run in 2012.