I think the main reason Obama has decided to forego public funding is that he sees an opportunity both to raise enough money to campaign even against rightwing 527s and to pull together a resource - a mailing list, if you will, of millions of interested supporters who can be directly contacted on a moment's notice.
But I wonder if there's not another angle to this move by Obama. Mitch McConnell has led the Republican opposition to campaign reform ("Limiting money limits free speech!") for years. Might not the prospect - and ultimately, the reality - of a huge monetary advantage by Obama during this cycle act as a forcing bid to get the Republicans to support significant campaign reform? They would never support it while they had the advantage; might they not change their tune now?
This is pure speculation - and it remains to be seen if a President Obama would follow through. But it strikes me as at least a possibility.