Sure, feelings are feelings, even Guy said it in the Show Me How to Play video (I think), that it was kind of crazy to see Mark and John rake in a lot of money during the tour, whereas he (and the rest of the touring band) got paid a musician's salary according to the contract, which apparently wasn't that great. But business (contract) is business, and as long as the terms of the contract are fulfilled, no one can complain. But that he was left in a great emotional vaccuum (oh, man ... Dr Ruth next?) is understandable.
However, come to think of it, I don't think Jack did anything special during the tour. He added some life maybe with the silly sunglasses and using no shoes at times, but sometimes it was really corny. I mean, look at Live Aid. It's like a band of electronics engineers + Elton John ... His whole rockstar approach just seemed out of place.
Another thing is that relationships change over the years. People move on, move places, get new interests, and contact becomes more infrequent, just like for all people. Maybe Mark and Jack were very good friends, but when they did On Every Street, it's probable that things could have changed. We're talking five years after the Brothers tour, Mark had done a lot of things, DS was inactive, Mark had kids, etc.
To me it isn't obvious that Jack should have been asked back. And that he now starts to "whine" about it is pretty pathetic I think. He should start talking about himself as a touring musician, not a rockstar. Then maybe reality would be easier to cope with.