From a process perspective, photos of an MPC receipt work as well with the MPC receipt scanners used by CBP as the receipt shown in the MPC app itself. For those who lock up most features of their phone and want to try to limit access to the device's functionality even as an MPC user, this can be very useful. Also, for people with more recent versions of the app that include in-app purchase features under the "plus" aspect of the apps, you could have one person pay for the app, use it for the "plus" features but share its results via messaging the screenshot of the MPC receipt to the person(s) who don't pay for the "plus" features while having the new app or who don't even have the MPC app.

And yes, I've used MPC with screenshot-captured photos of the MPC app receipts, some of which weren't even generated on the phone presented to CBP for MPC purpose.