I've just replaced the battery on my Lumia 820 after 9 months. They don't last forever. Glue is inexcusable. If they had a half decent mechanical design, foam pads would secure it in place when the device was assembled and allow replacement.
I've had a 4S for two years and have not noticed any problems with battery.
Perhaps the problem is the crap battery that goes bad after 9 months, rather than the way it's attached.
Glue is perfectly fine by me - and by most people apparently.