I don't listen much to tour recordings these days, especially those from shows I haven't attended. There has to be something quite unusual about a show for me, so I haven't heard "Summer Days" but the "boogie wooggie" description makes it seem similar to previous outings of the song - in which case, Mark should definitely not hold back in any way. It is an opportunity for some of his fast finger work and, IMO, Bob will respond to this, maybe squeezing a bit more out of a harmonica solo or some such. Dylan likes the spontaneity of the moment and Mark could seize that moment. I suspect, given his penchant for extensive rehearsal, he's just being careful and easing himself back in.
And, sweet surrender, don't blame Mark for not appearing with Bob at the shows you attended. I don't doubt that it's been Bob's decision. I can't imagine that Mark, if he had been asked before, would suddenly pop up in San Diego and say, "I'd like to play on Summer Days tonight, Bob. OK?". My own reading is that the two bands have been leapfrogging one another around the cities and venues for a few weeks but are now settled in Santa Monica for a few days, close to Dylan's home (well, one of them and a place where has a home studio) and in a town with lots of musical facilities (his latest album, TEMPEST, was recorded there) and a lot of music luminaries and friends around. An ideal place for a bit of minor grandstanding. We shall see.
Oh, and I don't buy that "it's the sound engineer's fault" nonsense.
