My guess -- and it's just that -- is that they had to move more of the electronics to the adapter side. Otherwise, why make the plug bigger and in the failed T shape. As you say, it's a step backwards. The original Magsafe was junk. I had two of them completely fall apart due to a poorly designed strain relief. The revised one that ran along the side did not stress the cable and so it wasn't prone to fraying. I love my RMBP, but agree the Magsafe2 is not a welcome part of that package.
Edit: OK, I'm always amused by the negs, but the one on this message in particular is interesting because I didn't even write anything controversial. It's a fact the original Magsafe T connector was a failure. It's 1) why there was a safety recall on them b/c too many came apart too easily and 2) why the it was redesigned to the stick form factor.
I never had the stick type fray on me, never saw complains on any of the message boards. I guess someone really woke up angry. Ha.