I'd welcome back MP if it would feel right for them. And if it was not a money thing, or a stunt.
Let's say that MP eventually patches things up with Myung and LaBrie, and they get all together and hug and say "ah, screw the past, happy carreer to each other", and then eventually Mangini leaves of his own accord and in a serene decision with the others... then I'd be all for it.
If instead it's all a "Damn, we need more exposure, let's just bring back Portnoy. James, call Mike and sort it out, he's back in, and someone let Mangini know he's out"... I would be quite bitter about it.