If the switches are off there is no current flow through that section of line giving you a dedicated line.
Whether or not dedicated lines are an advantage depends mostly on the amount of power you have to draw. Most systems will do just fine on a 15 Amp line. If you have multiple large amps that can pull more than 15 amps then a dedicated 20 amp Line makes a lot of sense.
Any noise at the breaker box is going to find its way into all the lines dedicated or not. You would have to isolate those lines that are critical or isolate the equipment distally. There are a host of devices for this purpose. I do not think there is much benefit for amplifiers more for line level and lower equipment. The most sensitive device is the phono stage. I love the way Channel D handles the problem by running the stage by battery and disconnecting the power supply when it detects a signal. After 10 minutes of no signal it reconnects the power supply.
Digital equipment is immune to extraneous noise as far as I can tell as long as you do not involve an analog section. I have a Lynx Hilo and nothing seems to bother it.