Most people don't change it, plus Apple went ahead and got drunk on control (UX consistency!) and on the idea that hardware is just the physical part of a service (experience!) that they provide.
People wanted them ultra slim, the old one was great because it was a pure rectangle, even had a charge indicator on it.
My 2013 macbook air is plenty slim and light but changing the battery is just a matter of undoing a bunch of screws and clipping it in. Takes <5 min. I'm not sure there are any real world advantages from glueing the thing in?