The only time I'm using one is when I'm recording. It's good to have it 'cause you can match everything. If it isn't used while recording I'm almost sure I can't get everything matched. I'm usually using two or three rhythm
tracks for guitar and two for soloing and because of that I can hear immidiately if the timing isn't exact on one of the tracks. It's very easy to slip out of beat without even noticing it....
While playing with a band it isn't that necessary though...