That's probably true in an electrically quiet environment, but shielding the cavity, or using shielded cable will lower the noise floor under almost all conditions. In the harsh environment of the average club musician, it's well worth the extra effort, especially if you have single-coil pickups. Why not take every advantage you have available?
You are correct. Especially at high volumes, a little noise goes a long way and shielding will help. I think people overlook proper grounding. No amount of shielding will eliminate noise in a poorly grounded guitar.