I do not get it. with the motor all you need to do is point to your true south sat which rarely is exact , i mean , coincides with your exact longitude. in theory you could even just using usals locate any sat and it will still work for the rest. just that probably the true south sat or any closer one is the easiest to find.
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I understand what you are saying, and you are right that you don't necessarily have to use the true south satellite, however it makes it a LOT easier if you do.
However I think you may be missing the point that while you don't need to aim the DISH at your true south sat, it is ABSOLUTELY necessary that the MOUNT be aimed to true south, and the sun can be a handy way of accomplishing this aim. I used to put a yard-stick along a flat surface of the mount/motor parallel to it's aim, and align this with shadows at local noon. However, it doesn't matter if you do this during the solar outage period or any other time of year. In fact you get best results in the winter when the shadows are long. You can use sats that are away from your true south to align your mount, but only if you motor the dish via USALS first then find the sat, via moving the whole mount on the pole, and this only works if USALS works on your motor. USALS doesn't always work, particularly if the motor's zero is off. If the zero is off considerably, the sun can be a great help in finding that sat. Plus, if both your south pointing AND your elevations are off, using the sun can at least get you pointed in the right direction, then you can find the sat with the elevation/declination adjustment.
Also, I've used the sun another way. Even after you have the mount aligned, the sun can be used to find satellites when using DiseqC-1.2 and/or a BUD motor/actuator). What I've done with an offset dish is to run a string from the LNBF up to the top center of the dish, then when the solar outage time comes, or the sat and sun have same azimuth, move the dish via motor till the string's shadow goes down the lnbf arm or across the center of the dish.
But yes, the main use is for site survey, to insure that you aren't blocked by a tree or something.
BTW, this is probably obvious to most, but if you look at the solar tables, such as those at the Naval Observatory, or astronomical programs, etc. You'll see that during the solar outage period, the declination of the satellites are approximately the same as the declination of the sun. Ie for most of us in north america, it's when the sun is about 5-6 degrees south of the equator, rather than around Sept 20-21 when the equinox occurs. Solar outages only occur at the equinoxes for people near the equator.
