I agree, I think it's financial. If you can keep the center piece and don't have to muck about transferring data, people may be more likely to buy 'themed' Magicbands. Not to mention the other accessories that go with the pop out piece. It's my understanding that the MB accessories they had originally didn't work well, so this is their next attempt at it.
Do the new bands need battery replacements? On the current bands, the battery dies and you can't use it for everything, and the batteries are not designed to be replaced. That means you need a new one every year or so.