Are the sampling and sequencing capabilities of the Fantom ever useful in performance, or are they used only in building up compositions in the studio?
Extremely useful. My band uses MIDI sequences played via my Fantom X. Our entire set is on there and provides all the instruments sounds except for guitars, percussion and vocals which we (3 members) provide. More details on our band site. Many (most?) professional bands work with sync'd MIDI accompaniment, with one variation being a real drummer playing along using a click track.