The standard-def HBO showing definitely uses much more picture information on top and bottom than the Blu-Ray does. They just cropped the little bit off the sides from the 1.44:1 to get it to 1.33:1, so it shows more than the Blu-Ray's 1.78:1. I don't know if the fullscreen DVD is the same, but I don't see why they would make a completely separate version from the HBO version. It makes no sense.

And no, I don't think creating a version for 4:3 TVs is pointless, especially if you want to recreate the IMAX experience of a shift from 2.39:1 to 1.44:1 as closely as possible. No matter how big your screen, a shift from 2.39:1 to 1.78:1 is nowhere near as dramatic a shift as the original IMAX version was. And switching from letterbox to pillarbox doesn't give the same feeling as a sheer "opening up" of the aspect ratio to fill the screen entirely.

And Transformers 2 is a different beast entirely, since the IMAX cut actually featured differences in footage (extended fight sequences, I believe), while The Dark Knight's 35mm and IMAX cuts are exactly the same with the exception of aspect ratio.

There are other things to consider, like how the footage that would be restored in a 2.39:1/1.33:1 version would mostly be excess headroom that was cropped for the Blu-Ray. As awesome as the dramatic aspect ratio shift originally was, the headroom was fine in an IMAX theater where you didn't look much at the top third of the screen anyway. The Blu-Ray version is better framed, and thus is probably a better home viewing experience than a full-IMAX-ratio version would be.

My point is, it could easily be done, but whether it needs to be or not is up for debate.