Every time I ask Apple users about the non-replaceable batteries, their reply is **always** -- (ie. without fail) "you just don't get it", without **ever** being able to articulate exactly what I don't "get".
For the most part, it's an aesthetic choice. For example, a battery door would interrupt the smooth surface of an iPod. The device wouldn't feel quite as nice to hold in the hand. The cost of that aesthetic is a more difficult process to replace the battery every three years or so. Is the exterior aesthetic worth the triennial inconvenience? I dunno. Maybe? It seems like a pretty subjective question.