It depends on the individual. They are [i]all[/i] unpredictable, however you want to interpret that, but if your Reveler has fled the Cult her/his lucid moments should outweigh the insanity. Otherwise... Well, she/he would be a danger to the rest of the team.

EDIT: Insane magic would simply be unpredictable as well, prone to bursts of power and complete lack of control. It would mostly be TK magic, more than offensive spells.