That's not the problem, the problem is when people attempt to use corner cases to destroy a good idea. Having a valid metric of when corner cases become truly important is a great way of avoiding people destroying ideas before they have a chance to come to fruition, simply because of corner cases that may or may not have major impact.

In the article he even states that it's good to hear the corner cases. You just have to make sure they don't rule the day unless they actually will have major impact.