The main point of replaceable batteries isn't being able to carry extras around (though that's nice too). It's being able to replace the battery when it inevitably dies. The battery technologies in current phone batteries are only good for 3-5 years before their capacity drops to unusable levels. If your phone's battery is replaceable, when this happens, you can...replace it. If it isn't, when this happens, it's time to buy a new phone.
Of course, us horrible cynics can't quite shake the notion that the manufacturers see this as a feature, not a bug.