People should only have strong opinions because they have convincing evidence. The point of the statement is that despite the fact you have convincing evidence

People should only have strong opinions because they have convincing evidence. The point of the statement is that despite the fact you have convincing evidence from your perspective, you need to consider the possibility that your perspective is biased and that your opinions maybe invalidated as the problem scope and externalities evolve. It's very hard to work with people with strongly held opinions, especially when there are more than one such people in your group.