Oops, oh yeah, I was thinking they just wanted to add drums rather than a live drummer. Well then they have just the same situation as my band does playing to MIDI files. Our drummer copes very well but you are right when you say it is not something all drummers are able to do easily. Basically they need a strong click track either in their drum monitor or headphones to keep them in time with the sequences. We do it using the drummer's monitor speakers. And because he plays a Roland VDrums kit the sound from his drumming doesn't drown out the click.