You're both right.

If you want to use the thermocouple, you'll need the amplifier to bring the signal up to a stable and readable level.
If you don't want to deal with the extra circuitry, though, you can certainly leave the thermocouple disconnected and install a thermistor, for standard operation.

Personally, I don't have the experience with 3D printers to make a call on this, but from an electronics standpoint, the fewer circuits and signal modifiers you have, then easier it is to setup and troubleshoot.

I don't recognize your printhead, so I'm not familiar with the layout. Would it be possible to remove the thermocouple and install a thermistor in its place?