Q3 being a budget version, it’s probably cheaper and has the advantage of swappable head straps as well as letting you rest your head on the couch or bed. It does move the weight closer to your head at least, but Vision Pro does the same thing. They decided to rather go for an external battery than putting anything in the back.
I’ll most certainly put something heavy back there to balance it out, the best thing for comfort you can do. I‘d definitely love to have the battery itself there, but I see why they’re doing it this way.