In the 2006 film "Perfume: The Story of a Murderer", there are several scenes depicting a bridge in 18th century Paris, with houses built on top of it: http://i.imgur.com/x54FS9k.jpg
edit: the title should spell "European" instead of "Europe"; sorry about this.
This indeed happened. I personally can't speak about Paris, but the old London Bridge, the one from the song "London Bridge is falling down" was entirely built up. Indeed there was so much commercial activity on the bridge that taking a ferry was quicker than going over the bridge by horse. Another famous example is Florence's Ponte Vechio, which still has shops on it.
I think collapse in itself wasn't necessarily the greatest risk, but the increased chance and increased impact of a fire on the bridge.
Medieval cities had few bridges, so they were very busy with traffic, and therefore building shops on the bridges was very attractive.