people use the sequencers to ADD TO the band, not to play things the band can't play. there is just so much done in the studio that you'd have to have a gigantic band to cover, so it's easier to let the tracks/sequences cover those things and let the band handle the big stuff. believe me, i've played with plenty of people using tracks, and it's just to fill out the sound.
2 good friends of mine play drums and bass for that group Rizen, and when they first started playing for them they just had them, organ and keys...tracks did everything else.
whoever said something about click/loops for tempo was right on as well, i use them for every song we do at church, just to keep everyone locked in.