There is one advantage of working in the metric mode that is not very obvious. It has to do with the least input increment of the machine tools in the selected input mode. The least input increment is the smallest departure movement possible on the CNC machine. for most CNC machines, the least input increment in the inch mode is 0.0001 in. In the metric mode, the least input increment is 0.001 mm. When converted to the inch mode 0.001 mm is equal to 0.00003937 in, so 0.001 mm is less than half of 0.0001 in. This means the machine has a much finer resolution or movement grid when you are working in the metric mode. You can target the end point of each movement command to a more precise position when working in the metric input system. 

Metric mode

Inch mode

We are not saying that the machine is more accurate in the metric mode. The CNC machine will perform to its quoted specifications in either mode. In the metric mode, you can simply target your end points to a finer location. Compare this to an indexing device mounted on the table of a VMC. If we have a 5º indexer, we can specify angular indexes every 5º. If we have a 1º indexer, we can specify angular index every 1º. The 5º indexer will have only 72 positions, while the 1º indexer will have 360 positions. With the 1º indexer, we can target angular end points to a much finer location. However, this fact by itself does not make the 1º indexer more accurate. In a similar way, the metric mode will allow the possible end points along a liner axis to be more than doubled. There will be 2.54 times the number of end points for any linear axis in the metric mode than in the inch mode.
For a linear axis that is 10 in long, there would be 100,000 possible end points in the inch mode. For the same linear axes, there would be 254,000 possible end points in the metric mode.