It is my opinion that singers are, inherently, rhythmically lazy. I think instrumentalists spend too much time counting to be rhythmically lazy, but something about singing words gets in the way of counting numbers.
Witness the career of Robert Shaw, whose many achievements included the “accomplishment” of getting singers to be rhythmically precise using count-singing.
More recently, with the advent of pop a cappella, we’ve found an easier solution to get singers to be rhythmically accurate: outsource it. Get a vocal percussionist (you can even make it an actual percussionist!) and let her be in charge of the rhythmic vitality of the ensemble. Everyone else simply has to link up to this one rhythmic source. (Note: this is not easy, but it is easier.)
It is my opinion, though, that, while difficult, it is far more rewarding when an entire ensemble commits to overcoming rhythmic laziness and engages in the vitality of the rhythm. It takes concentration, repetition, and physical engagement with the music. But when it happens, a choir can make rhythmically compelling music across any genre, without outsourcing it to one source.
When the singers all overcome their inherent rhythmic laziness and commit to the beat, the music comes alive.