The guest were there to enjoy a show like any normal person and they were not here to go at work.
Mark has no obligation to ask some of his guest to play with him, we can regret it but using words like "Unbelievable" or "scandalous" is way too much IMHO.
Of course he is not obligated. Thats why he doesn't invite anyone to guest, lol
It's not an obligation but a mattter of enjoy playing live with friends that usually dont play with as they live very far from you and coincide with them is difficult.
It shouldn't be an obligation but a pleasure.
But lazyness is stronger than anything else.