Locked out distributor...

As I'm preparing to install my MSD start and step retard controller pn 8987 it recommends the distributors centrifugal advance be locked out?..
I know I'm not going to do this likely but I was wondering why they would say this?
I get that the start retard is made for this kind of situation but I plan to use the step retard for nitrous...
Just wondering what street drivability and idle would be like at full advance? Or if it's even possible? I've seen cars at the track like this but that's the only place...
Thank you for your knowledge and experience...