"Switch computers often" can also apply to "switch computers with little notice". Even if 95% of my time is spent on one computer, it's nice to know my config is safely squirreled away and, uh, trivially unsquirrelable if something terrible happens to this hardware and I have to get another computer. Seems like a relatively low probability event, but my child has already destroyed two ThinkPads (both were very old and very disposable--still an accomplishment).

As to your last question, nix+home manager gets you there, but that's a whole other Thing.