If they are so worried, they can just make an alterative character who occupies a similar party role. E.g. a cleric and a bard.I can imagine some parties getting very concerned over party balance. Epically if they can't control which character they are running at a given time or if they don't all switch characters at the same time etc. Would be easy for a party to end up with no martials, or healers, or spell casters, etc at a given time. I would see that as a challenge, but some would not like it.