Dual spindle milling machines are preferred for several reasons:

1. Increased productivity: With the ability to work on two parts simultaneously, dual spindle milling machines can significantly increase productivity and reduce cycle times.
2. Reduced setup time: By having two spindles, the machine can perform different operations on each spindle, reducing the time required to set up and change over the machine between different parts.
3. Improved accuracy: The dual spindle design allows for greater accuracy and precision in machining, as the machine can hold parts securely and reduce the chance of movement during the machining process.
4. Cost-effective: Dual spindle milling machines can be a cost-effective option for mass production of small to medium-sized parts, as the machine can perform multiple operations at once, reducing the need for multiple machines.
5. Greater flexibility: With the ability to machine two parts at once, dual spindle milling machines can be used to machine different parts, different material, or different finish on each spindle, increasing the flexibility of the machine.

Overall, dual spindle milling machines are preferred for their increased productivity, reduced setup time, improved accuracy, cost-effectiveness and greater flexibility.