I wonder if this change might indirectly improve operational reliability by improving weight and balance? Or maybe somebody at B6 with similar weak knowledge of the matter thinks so?

I've been on plenty of B6 flights where the back of the bus was pretty sparse with a packed front Core section.

For Mosaic's anyway, it means that in the unlikely event EMS ever fills up, at least you'll get a seat near the front. For everyone else, its like a $5 lottery ticket to improve your chances of having an empty middle seat next to you.