I honestly cannot contribute a "con" of brainstorming, as I believe it to be one of the best practices in learning, both for teachers and students.
In addition to the above comments, one thing I have found with brainstorming (both for myself and with my students) is that it seems easy and fun. There is no "pressure" when it comes to brainstorming. I'm not sure when the excitement for being the student who knows all the answers wears off, but it is certainly sometime before 9th grade. If it weren't for "brainstorming," as a high school English teacher, I might never receive a response from over half of my classes. As soon as I label something "brainstorming" however, everyone feels free to participate without fear of being wrong.
It works the same way with teachers in planning. We tend to come up with better ideas when we're allowed to (and comfortable with) get all the bad ones out first.