Difference is, you can bring in a guitarist and still play 25-30 tracks without a problem.
If Jon's knee is giving constant pain, then the "show" as a whole isn't as good. Jon won't be able to give 100% to the performance and there is the risk of making the knee worse.

I know I'm not going to any of the shows, but I'd rather have a refund than have a set where the front-man can't give his all and end up with a shorter, more vanilla set.
Unless he gets dosed up on morphine or sits down, I can't see Jon managing a 3 hour show.