Some, maybe. Others live perfectly happily without being alone in their heads. I know a story or two about people for whom just talking with the voices was the way they found themselves again. Sure there are people who can't deal with another constant presence or who have deeper issues to work out, but saying that everyone who's part of a plural system needs institutional help is part of the feedback loop that makes them reject their headmates and so need institutional help finding solitude. The key is finding a way to a stable life, not finding a way to a single life.