When you arm its shown with LED signals on main board and external signalling. The GUI says "armed" and prints a message in the console and the ESCs give a beep and the LED on them goes green.
If after that you are not aware or forget that you just armed your craft, I would advise to reconsider if handling dangerous equipment like an MR is an ideal path for you....
Having the motors spin when armed is in my personal opinion completely useless and a total nuisance. And if I push the argument a bit, I could state that its also dangerous, since the time it takes to stop motors completely gets considerably longer than the time it takes to just lower the throttle stick to 0.
As I remember, it originally came from the way Multiwii made sure that a minimum throttle level was always maintained to prevent the motors from stopping in flight. The intention was never to create a way of showing you the craft was armed, but to make sure ESC´s would never go below a predefined level.
And in multiwii, the option to disable it was present from some of the very first releases - why? Because a lot of people found it annoying and asked for a way to disable it.
I think the sentiment in most of the team is that its not needed so its not been proritized, since it means adding more parameters and we have a lot of other things we are prioritizing to do with the ressources we have.