I debated the same thing. In the end, I decided against it. I would love to see another FULL ON headlining tour, but this co-headlining thing they've done the past couple of years with Motley Crue and now Def Leppard is a bit of a turnoff, simply because I am a greedy a-hole who wants 2 hours of Kiss, not 90 minutes. I'd love to see Motley Crue and Def Leppard as well, but I also want a full concert of both, not some opening band / co-headliner thing.
I last saw KISS in 2010 when they played for just over 2 hrs and 20 minutes. Easily one of the best concerts I've ever seen and they even had a small stage to deal with, as opposed to the normal monster-sized stage.
I do wish that I had just gone ahead and purchased the tickets, I know I wouldn't have been sorry (other than the price which is getting pretty spendy).