Lets talk dedicated lines

About a year ago I installed a dedicated 20 amp line for my system...and went as far as modding the whole power structure from the street transformer to my panel to my house...at some fairly major expense. The result was certainly an improvement for my system.
Recently i got to thinking that a lot of us have a system that utilizes the standard 15amp line that more than likely only supplies one other component in our rooms...and that is the lights. While lights can be noisy on the system, particularly if they are halogens, then this is a source of considerable hash and noise. However, since the only other thing shared by this line are the lights...and not the more power hungry grabbing stuff..like the refrigerator, or the Ac etc, then simply playing the system in the dark should essentially allow for a dedicated line effect to materialize on the gear that is attached to this line as well. The standard room convenience outlet( which is what we are talking about here) is 15 amp breaker protected and is only supplying the lights and the convenience outlet in a room....so ( assuming that this is the case--and the lights are the only thing supplied besides the outlet) what is the real advantage to the 'true' dedicated line ? Twenty amps is more than a lot of auto gear needs, so I doubt that is the issue...thoughts?