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Just to pick up on some points...

Hardword or Aluminium.... journal bearings? are you sure?

Journal bearings at the accuracy that you're looking at (3 micron) is a job for a rigid lathe and...

Many reasons for a high Z-axis but no right or wrong for either, it's all down to usage.
For instance, I recently built a machine with a lifting gantry that had 400mm of Z-axis travel but was never...

Similar to Clive’s response you might want to drill holes in the end or side of say a rectangular piece of material so need to have space under the tool.
As mentioned being able to move the spindle...

There is sometimes an advantage to being able to move the spindle up and down on the Z carriage a bit to accommodate different cutter lengths provides you have clearance.

Rotary machining,for a start,for clearance.

You might have a part 150mm thick and only want a pocket 5mm deep. :cower: