I have done this! I currently carry a GPD Win Max 2 around, but let me be clear about some elements of this: the keyboard on a sub-device is just as important as any other, and the keyboard on GPD devices is not great. A sub-device has to be maintained and charged and given love and care just the same as any other device. The amount of times I have found my cute linux laptop dead when I wanted it, outdated, or randomly signed out of services is quite high (and this is with an onboard LTE modem). Carrying around a folding (or even non-folding, like a Magic Keyboard) is lighter than a sub-device, will last significantly longer in terms of charge/battery, and does not require any of the secondary device maintenance that is needed with one normally.

To me a folding phone is an absolute no-brainer in terms of why it is useful. You can get a very, very good keyboard for cheap, or try many, and completely eliminate the need to carry a second device.

Another huge use case I have is transit/travel video playback. I do not normally want to bring an iPad or even a secondary laptop during travel, so this effectively deletes one more device that I have to charge, update, bring adapters for, etc. It is effectively all of the benefit of having a tablet at all times, a sub-laptop at all times, and requires absolutely no additional charging or maintenance.