it's their record, ask them. if they want consistency or if they want a live feel.
Sorry man but as long as I've been doing audio, artists are usually the ones making the bad decisions. Their record or not, your name is on it too. Most of the artists that would not use one are the worst offenders of having bad timing.
the problem is too many people confuse "live"/"natural" feel with sloppy and inconsistent timing. good drummers can still push or pull the groove within the confines of a click track, and if that isn't enough, you can program subtle changes into the tempo track - 1bpm ramps over the course of a 2 bar fill leading into the chorus, pushing the final chorus up by 2bpm to give it abit more excitement etc. get it all sorted when doing preproduction/guide tracks, make sure the drummer's present during the process and paying attention..
i've had many drummers who have never played or practiced with a click ever and were understandably terrified of using one in the studio, but once we figured out the right tempos for the song everything just falls into place.
This. Just because there is a click doesn't mean it can't have a live feel, or groove, etc. It's about having consistency.