Me too, for many reasons:

-It would be labeled (wrongly) as the Ultimate Hair Band Tour. Neither band would go for that.
-It would stink of nostalgia, which we all know how Jon feels about.
-Def Leppard would have to be the opener, and I don't see Joe Elliot going for it.
-Sorry folks, but Jon would be put to shame by Joe (and he knows it.) Joe's voice has gotten better over the years.