This is the question asked by Brand Republic, with the conclusion: "it might be time to start coming up with some reasons for clients as to why they shouldn't turn their marketing problems over to the hive mind to solve."
To me, there is a huge difference between idea generation and creative development.
A great idea can come from anywhere. Agency people have always known this. While Creatives are the ones tasked with the job, client services people, clients, even admin staff have been known to chirp up with excellent ideas when given the chance. Sometimes entire campaigns are built around conversations that focus group members have when the moderator leaves the room and they forget they are being watched. Just the other day, my Kindergarten-age son said something in the bath that became a concept for a commercial. Even spoof ads from Adbusters can inspire the very people they mock.
But having ideas is not the hardest part about being an advertising Creative. Knowing which ideas are right for the job is.
Throwing your brand open to the ideas of the masses isn't that much different than training intern Copywriters. You get back all kinds of crazy and inappropriate shite, but you can sometimes sift gems out of it. Without strategic guidance and Creative Direction, however, they are just ideas. Concepts are those ideas that pass the creative brief acid test of brand, objectives, message and audience, and can be executed effectively on budget.
So go ahead and get your ideas from adhack if you like. Run a contest to get user submissions for "reality advertising". But you'll never get rid of the admen. Just as with reality TV, the storytelling is all in the editing.