audi:Unless it's something that's actually cool, I really don't care for fans getting on the stage. It is a thrill for no one except the people onstage and their friends.
As for everyone else, the show which they'd paid good money for has come to a lengthy pause.
Again, if it's an actual cool thing, I'm good with it: Like the little girl who played bass on "Get Back" or the dudes in who made the Abbey Road replica and got Paul to pose barefoot with them for a picture.
At the very least, the audience was still being entertained.
On both last year's tour and this one, Paul has used words something to the effect of now it's come to that time in the show and then the fans go on stage.
I liked it much better when it was mostly done at sound check. That way it didn't interrupt the concert.
I did go to one concert a few years ago now, where the dad had a sign that said the kid could play Get Back. They got up on stage and the kid was terrified. Brian Ray tried to help him, but he just stood there with a blank look on his face. Paul gave the kid a big hug and off the stage they went.