The choral, or group, warm-up is insufficient for the serious singer, who must find the time for a pre-rehearsal, private, personalized warm-up. The choral warm-up serves other purposes well, in ideal circumstances.
Each singer should report to rehearsal already warmed up, ready to function as a member of the larger vocal instrument. To do less awards to others (conductor, assistant conductor, etc.) a level of control and influence that should be prized and safeguarded as an individual right. Concurrently, the singer who arrives at rehearsal unprepared to offer his/her best singing does not contribute responsibly. Everybody loses.