The only interest for me would be volume, although I tend to boost volume at different levels for each slot and so not much of a need for this to be "global".
For reverb and delay mix, for example, this is less useful for me because:
1) I don't maintain vast numbers of performances and slots and therefore having a global morph is of little advantage
2) I don't need multiple morphs - I tend to use morph as an "additional" slot and so morph multiple parameters at once.
Also note I only use Morph via the switch for instant changes, not through an expression pedal.
This is just me though, I can see people with a more complex use case having a need though.
I really appreciate the reach out into the community!