The basic reason for the advice Never Volunteer seems to be that when the worst tasks come up, ranging from undesirable to potentially fatal, leaders can ask for volunteers both to give troops a chance to show their spirit and to somewhat reduce the effect on the leaders' consciences for making choices that could result in injuries or deaths.
Volunteers in any situation usually perform better than those who are drafted (in the large or small sense), so asking for volunteers is often a wise thing to do in many circumstances, good or bad.
Situations not being good times to ask for volunteers are any that would make the leader look ignorant (not knowing which of his troops do which tasks the best), indecisive, or cowardly.