Lack of diversity may be a problem because then you've got a lower chance of getting the right answer somewhere in there.
That, too, but there are other issues as well -- e.g. risk management through diversification.
Otherwise, what could be thought about that doesn't have a correct answer?
Is she pretty?
Should I be a vegetarian?
What's the best way of tackling that problem?
A disagreement on any of those questions reduces to either incorrect reasoning or differing preferences. People having identical preferences may be uncommon, but I don't think you can say it means someone isn't thinking.
Another month, another rationality quotes thread. The rules are: