and at one point the physically matching connectors with different pinouts meant Dell PSUs would fry regular motherboards or vice-versa.
I think it's more than "uncaring" - since they just shifted the pinout 3 pins over from standard ATX, got rid of the 3.3V and added another 2 5V lines: https://www.vogons.org/viewtopic.php?t=59959
I think dropping the 3.3v and -12v connections is a valid choice, but they probably should have found a different connector to use so the incompatability was obvious.
They could have also dropped those rails by just dropping them, and leaving the connector the same. Itβs pretty unlikely they needed the extra current on the 5V rail.
> I think dropping the 3.3v and -12v connections is a valid choice,
More than just a valid choice, in the long run even 5V is going to be dropped. Intel's ATX12VO standard has only 12V coming from the power supply, all other voltages are generated on the motherboard when necessary.
I think it's more than "uncaring" - since they just shifted the pinout 3 pins over from standard ATX, got rid of the 3.3V and added another 2 5V lines: https://www.vogons.org/viewtopic.php?t=59959