the power/signal noise myth is believed to come from the magnetic field surrounding the power wire created by the current flowing through it, but to make a long story short - it doesn't hapen.
I guess all these shops are doing it wrong then,"rpm" noise can be caused by different things, amp ground,bad rcas, alternator and yes rcas being ran next to the power wire.
"Well...some guys I know do it this way..." is not exactly factual evidence.
The ONLY issue with power/signal close together is EMI and that has nothing to do with noise coming from the alternator or ignition system. That's a fact, and it's really not difficult to grasp.
Installers believe the myth because its what they've always known. The ones that take the time to learn some science know it's not an issue.
It's actually becoming more and more common to run power/signal/speaker/remote wires all in the same bundle for the length of the car.
If you still don't believe me - fine - we'll just disagree.