Well... Do you know about steering diode usages? Your board does nothing more than a SCSI ribbon cable.When the MotherX4 was built in 2013, I have spoke with some electronicial peoples and they recommanded to add it.
First, because it is better than nothing to protect the CPC... Next, because one compensate the other if it goes weak, instead of failling.
Some peoples use the external PSU to save the content of a RAM Drive, can turn off the CPC power without disturbing the expansions...
In fact, the problem is for your kind of "arduino" modules that require to be 3V3 powered and your 5V to 3V3 regulator should not have enough voltage from the CPC itself.
The DDI-3 was never intended to be plugged at the end of the MotherX4 without an adapted external power supply... But directly on the CPC464 edge connector.
Need more voltage? Input more voltage!
Please, stop to focus on the MotherX4 dioles... It is a solution for around 250 happy users.
The problem is your dangerous fix posted on cpc-wiki :

Returning the external PSU current into the CPC VCC line...
Please, clean behind your house first. Thanks.
@robcfg From my tests, it is recommanded to use an external PSU if you are using more than three expansions, to save your CTM life.