Right - when it's an emergency (hence the name) you need it in hardware, not software.
I wouldn't call it "immediate stop", because that suggests it's the same as an emergency stop. Also, it gives the operator 2 options, so if there's an emergency, they need to think and make a decision on which button to press (SW or HW), and decision making is difficult and unreliable under emergency situations. Stick with having a HW stop, and monitor it with a digital input so your software can react appropriately.