It's pretty commonplace, and pretty bad form I reckon. Biffy Clyro and Korn both have (or at least had) guitarists in the live band who weren't even allowed on the stage.
That reminds me of a Michael Schenker story from many years ago - he had a roadie called Steve Casey who used to play some of the rhythm guitar parts, offstage (one of the music papers I bought at the time had a picture of him standing behind the amps with a black Les Paul, if I remember right!).
When Graham Bonnet was in the band, he got very drunk one fateful night and pulled Casey on stage, saying "this is the guy who really plays the solos" or words to that effect. That was Bonnet's last MSG gig.
Given the state Schenker was in the last time I saw him, perhaps they should bring Steve Casey back....