He must be able to sing it live. He must be able to remember the lyrics. He must be able to play the guitar parts and do it parallel with the vocal parts. He wants to have fun with it, it should not be too hard work for him.
Then, each band member must be featured especially. You could find another song for Mike playing Pipes, but would that one fit with Mark? Jim is soloing on CBC and IUTC. He could also play on other songs, but would they also be fast and "rocking" enough for the set?
Guitar endorsement is important, too. For example, there are Martin songs needed like Marbletown.. and as this one features Glen and John at the same time, it will stay in the set forever. So building a set list that considers all this is not that easy. Plus guitar changes have to be considered, too. He needs more slide guitar songs these days as he isn' t able to pick for two hours anymore.. and so on and so on...
Mark is the opposite of spontaneous. He is full of energy when it comes to tour and doing it all, but I guess you could call him sort of lazy in learn, rehearse and remember older stuff again. He has to find Dire Straits classics he is still able to cope with on the guitar and feature the current band at he same time. So it is nothing short of a wonder to get a proper set list at an MK concert...
LE