That's also what I'm wondering. For my primary work PC it's: Either it's really portable and has a screen - then it should be small. Or it's not portable - then I don't care as long as it fits on or under the desk.

For some environments however small or invisible hardware and the associated clean look is definitely a bonus: E.g. if it's a PC in your living room (I bought a NUC for that), or maybe for publicly exposed working places (receptionist, ...). But that's not really what is targeted with that hardware.

On a 'living room' PC: I bought a Thinkpad T430s for £150. It's connected to the TV, the amp, and syncs various file storage services to itself then backs them up to various locations with Crashplan.

The best thing about it? If I want to do anything with it, I just open the lid. Screen, keyboard and pointing device built in.

Yup, lots of it. Smaller computer means more room for other machines, like larger CNC Mills.

As long as it meets the specs, most CAD people probably don't need expandable hardware.