That's a good point. But could you mitigate that effect by giving it a permanent spot, maybe like a signature in the corner? We could tart it up so it fits in. And if it's part of a bigger overall advertising effort across the city (incl. street furniture) then it might look less like a desperate fill ad and part of a bigger thing.
And by advertising I'm thinking of events like Pride and Caribana, and the bazillion festivals we host. Maybe something very simple like "Parkdale Book Festival, June 6,
www.toronto.com or whatever URL." Gives it a distinct Toronto feel that no other advertising wall in any other city will have and raises profile of cultural events in a place where lots of people could see it. (This may be a bit radical, but I don't think the City should be paying to advertise in public space that it allowed to be created.)
Perhaps I'm thinking more of the practicality and the opportunity than of the aesthetic. I make that mistake often when I dress in the morning.