If you can get away with it then sure, but once the illusion is shattered, the show is done, and that was the case with Eminem in 2010. It was blatant.
If I found out after a show someone lip synced my attitude would most likely be "f**k it".
Saw the Black Keys when they sub headlined in 2014 this is an absolute solid setlist. But yeah this was probably there peak - not at this level anymore - and kicking of about Glasto pay the other year will not have helped there case probably not see them again on the farm.