It's their thin-is-better lighter-is-more-important model. In the traditional 14-16" size, the T4xx/T5xx devices do what they are supposed to. A changable battery (while running, without tools) enables you to do things like swapping an empty battery for a full one, either by plugging in shortly, using a secondary battery in the case, or briefly shutting down. I don't want to carry that much energy with me all the time, but I still want the option to use the device without having significant downtime. A short break is fine, but that's 5-10 minutes, not 30+ for charging the battery.

I believe the hot-swappable batteries are a goner on the latest T series machines.