What do you do after the brainstorm?
You see, a brainstorm should be a beginning, not an end. Too many people see the event - the brainstorming meeting or the idea campaign - as the desired result, rather than just a beginning step in a process to build new products or services. Generating good ideas is not necessarily easy, but with the right preparation and facilitation you can generate hundreds of ideas, and rank or prioritize those ideas to identify the top 10, or top 20. What becomes more interesting, and adds more value, is the next step.
Once ideas are generated, you need to have a mechanism to consider them, evaluate them, and determine whether or not to create pilots or prototypes, and what the mechanisms are for commercialization. Without those subsequent steps, your ideation session is just creating ideas that for the most part will never be considered after the event. Probably one of the most important questions you can ask when you are invited to a brainstorming event is - can you describe for me how these ideas will be worked once the brainstorm is completed? In three months, what will be the result of these ideas? If you can't get a good answer to questions like these, then the session is either not well planned or the outcomes are uncertain, and both instances are problematic.
On the other hand, if you ask those questions and get confident, specific answers, then you should attend, because your ideas might be the catalyst for something even better. The important issue isn't the idea generation, but the work you do after the generation and how that work is managed and who is responsible for doing that work.