What a silly point. Saying that they "could have done X" to the Macbook Pro (where X is a crazy feature nobody has ever asked for) doesn't mean that taking away feature Y (where Y is a feature many users find useful) is a good thing.

The actual space/weight savings of going from removable battery to built-in are negligible. Two of my coworkers have unibody MBPs, and they're a bit slimmer, but I suspect that has more to do with incremental improvements in design than the removal of a little bit of materials for the battery compartment. (My Macbook Pro is a lot thinner than my old Powerbook G4, after all.)

Besides, if having a light, small machine is that important, you should get a Macbook Air. The Macbook Pro line has always been the slightly bulkier but more powerful line, where users are willing to trade a bit of size/weight for performance/features.