Maybe I'm just not paying attention to the right bands, but is this something that other bands do? Just bring back ex members for concerts (much less an entire tour)? I don't see Maiden inviting Beetle Bailey on stage to sing with them or Ripper Owens with Judas Priest.
Yes's Union was a slightly different thing in that it was a merger of two versions of the band with a single member common to both. WDADRu was a pretty unique thing to my knowledge, and it seemed a bit out of place to have Derek there.
Bringing MP back for more than one song, unless it's a last and final show ever (or an RRHOF induction - LOL), would just be a terrible idea that would have no lasting effect, except to send us back to the "which Mike is better?" debate that dominated the first half of the 2010s and which occasionally pops up now.