[kemiller]

I'm assuming you mean user-replaceable, since as others have pointed out, it is replaceable by a technician. And the answer is: it would compromise too many other areas of functionality. User-replaceable batteries would mean both internal housing hardware to create a bay, electrical contact interface, and housing for the battery itself. These would either add weight and bulk, or reduce the capacity of the battery overall, and it would make waterproofing much harder. I personally very much prefer the trade-offs Apple has made, and I suspect a lot of other people do too.