Just before a brainstorm someone typically says “there is no such thing as a bad idea”. Of course there is. I have run over a thousand brainstorms and trust me, most of the ideas put forth weren’t great; they made little sense, were strategically unsound, logistically illogical, and sometimes downright weird.
The point is, even though most of the ideas generated in a brainstorm are rubbish, they are incredibly valuable in the brainstorm itself. They unlock thinking. They are stepping stone thoughts that spark new ideas. They break the silence and they generate momentum for the group.
In addition, pointing out bad ideas during the brainstorm shifts the group from creativity to critiquing, which isn’t productive. It also attacks the person who put forth the idea, and stops others from putting forth theirs.
It’s also not much fun seeing an idea killed; and brainstorms should be fun, and wacky, and outrageous. You should end the session exhausted with hundreds of ideas on the board, because in amongst the rubbish will be some gems.