I purchased a Lenovo T580 ThinkPad in 2018, and I configured it with an extra battery. So it had one built-in and one that was removable. This added some extra weight and plenty of extra battery life.

Several times I thought about purchasing a spare battery, but I never did. I also thought about detaching the battery and using the machine without it, such as on AC line power, or when I didn't need the extra juice. But I realized that it was a bad idea to carry it around with a gaping hole on the bottom and exposed battery contacts. I would've felt comfortable if there was a dummy cap to snap on instead.