A couple things here:
Notice that I said that I'm off-grid
in Arizona. I've never dealt with APS, so I wouldn't know how reliable their service is on a personal level.
I am acutely aware the power for the WAPA Desert Southwest and WAPA Sierra Nevada districts (the areas I lived) come from. And I am also acutely aware of how it all works.
Sure, power is supposed to be 60hz within an extremely small tolerance. But, as I said, I have found that it is not always clean and not always at ~60hz. In the areas I lived "Sags", "Swells", "Rolling Brown Outs" and "Transient Impulses" were/are very common and cause a wide variation in the frequency of the power. And they happened quite a bit, every single day. PG&E and SCE say they only occur for milliseconds. But, the University of California has data that shows otherwise. This study from the University of California (LBNL) documents that there are many sustained frequency variations in the national grid. In fact, you'll see that there are documented deviations that were sustained for periods of one minute or more. http://www.nerc.com/FilingsOrders/us/FERCOrdersRules/Interconnection_Frequency_Performance.pdf
. Maybe my measurements were wrong but the fact that PG&E and SCE both say that their variations are just "occasional" and only last "milliseconds" while UC Berkley has documentation that shows hundreds of these variations lasting one-minute or more, I'm not convinced.
Of course, there was an article in 2011 that reinforced my observations that grid power is dirty. And it used clocks in various areas as an example of the deviation, pointing out that some of them are as much as 40-minutes off. It was an article about allowing a proposed year-long experiment to allow wider variation in grid frequency. That article is here: http://www.washingtontimes.com/news/2011/jun/24/ap-exclusive-power-grid-change-may-disrupt-clocks/?page=all
I realize that you said nothing about the other quality issue I mentioned, voltage being delivered to the customer but I thought I'd also address that. Conservation Voltage Reduction is quite real. The utilities (at least in California) reduce the power provided to homes. It's only supposed to be a ~5% reduction, but, again, I saw voltages much lower than a 5% reduction over very long periods of times extended periods of times. The utilities will say it's a misconception that CVR will not damage appliances, and it probably is true if the reduction is ~5%, but I experienced reductions in the 12% range. Does this cause interference? I don't know but I'd bet that it causes other devices to run at less than optimum performance, potentially causing interference.