some of the early performances of MJ - Like the Billy Jeans in Motown weren't performed live. it was a recorded track and he just lip-synced to it. because a lot of importance is given to his dance rather than the song.
there may be various reasons for not including singers like Sonu Nigam and Sukhvinder singh that we don't know. it could also be because ar like to give opportunities to the upcoming singers. given the kind of set up, the variety of songs performed, the audience interaction and the excitement, there is always a chance to forget a few lines or even go off-key a bit. come on, they are singing their hearts out for 2 and half hours. no one is that perfect. It is probably easy to perform a rock or jazz concert than AR's concert. when Bryan adams performs, the audience are only his fans or people who love that kind of music. but for an AR's concert all indians would go and even some foreigners. the level of expectation is even higher. Finally, there is nothing wrong with going off-key or out of tune here and there, that's what the live concert is all about.. there will be mistakes, and there will be terrific performances like the Mousam and escape. a concert is enjoyed as a whole not song by song, IMHO Ta