First off, you can replace the batteries on most Apple laptops ever made, including the 2009 model MacBook I'm typing this off, yourself. Secondly,newer models (including all 2010 models, all MacBook Airs and last year's MacBook Pros) don't have user-replaceable batteries for design reasons. By fixing it in the case, it lightens the thing up and frees space to have greater capacity- Apple reckons they should last for five years, and Apple offers a replacement battery programme for those models. You're not being forced to upgrade by any means.