adwsystems:
We are talking apples and oranges. I am talking power in. You are talking power out. Your complaint has nothing to do with what I am saying. I am saying to choose one of the three power inputs and you are "confusing" issues by including power output (ie., using power from the 5V pin). I never said to use the 5V power out.
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The 5V pin as a power source (out) is useless. On that we agree.
Yes, of course I am talking about "power out" if that is the thing that is required to complete a project as in this particular case. If you need 5 V for a purpose and the Arduino cannot be used as a source of 5 V, then a power source that will not provide that 5 V is simply not relevant; not an option.
Your stance is very peculiar, since you appear to keep criticising the use of two separate power supplies, but powering the Arduino via the regulator at 9 V mandates a second supply for the 5 V yet you are suggesting that it is "possible" to use a 9 V input.
adwsystems:
How many devices out there run on 9V?
In my experience, most cordless phones, older telephone modems (when these were a meaningful device) and ADSL boxes, network devices, Ethernet hubs, toys, radios and many others.
These are the things that were contemporaneous with the original Arduinos, so Arduino made use of the available power packs.