There's another good reason to leave the disc on, especially with mtb or off road use, even if you're a great mechanic and everything is always perfect.

When a bike falls or is crashed the RD hanger can get bent. For obvious reasons it's always inward. When the hanger is bent, all the RD settings are offset inward, including the limit screws. That means that an RD was set up correctly can now overshift into the spokes.

Granted, if you're alert, attentive, know your bike, and care for it, especially for road use you can live without the disc as many do and have no problems, but otherwise it's cheap insurance against expensive damage.