I can do spindle speed control on my machine but I never use it. I probably would if I had to do series production of the same part but for my one-off projects (rarely small quantities) I want to have manual control.

The rpm range is anyway limited. I usually don't go below 9k rpm because the spindle power/torque goes way down and not often beyond 17k rpm except for the 1/16" end mills. In between there I set the spindle speed by ear. You can hear when the machine is most happy.