If the rules are unfair and can be exploited, then I think that's an issue with the rules, not the players. As long as players are playing within the confines of the rules, I think it's fair game. Calling it exploitation is just loaded language, in my opinion.
Americanarama last year comes to mind. Dylan wasn't even bad, but he was almost like the after-show.
Also, both times Robert Plant has pre-headlined Bonnaroo, if you can count that.
That whole show blew me away from Richard Thompson to MMJ to Wilco to Dylan. Wish the Indy show I was at featured more of the collaboration of later Americana shows on that tour. I could understand picking that show for this thread though.