If you don't have the 3-disc CE3K DVD, then the Blu-Ray is still worth the upgrade.
And The Dark Knight's shifting aspect ratio isn't OAR - IMAX's aspect ratio is 1.44:1 (preserved on the IMAX Prologue bonus feature on the Batman Begins Blu-Ray), while the Blu-Ray crops this to 1.78:1 (16:9) to fill a widescreen television.
The 35mm scenes are shown in their proper 2.39:1, though.