Once a city has created an Open Data Framework, and provides data related to the city the whole point is to stimulate third parties to leverage on those data for reasons that are not related to the ones that have led to the production of those data. As I noted by far data are being produced for very specific reasons and the decision to make them available is in no way decreasing their original intended use.
As an example luminosity sensors are deployed to switch on/switch off lamp posts and save power whilst providing illumination when needed. The data (on/off) can be shared to anybody and I guess someone may find a way to exploit them for different reasons (like, just for the sake of reasoning, determining the presence of cars that can be derived from the reflectivity of their surface, higher than tarmac).
It is usually not evident, beforehand, to identify alternative use for data. At the same time, experience shows that if you get a sufficient number of people you will end up with unexpected, and often, bright ideas.
A "smart" city therefore should go beyond the opening of its data, and soliciting third parties to make their data available. It should create contests, set up idea challenges, involve schools to stimulate out of the box ideas on leveraging on data.
There are already quite a number of companies that have realised the value of accessing data to offer services. In the figure an example: Colouree is selling to municipalities services to emphasise how data can be leveraged, in their case for better planning.
Amsterdam has opened up the parking data (acquired by scanning car plates as a more efficient way to have parking payment) and TomTom has included this information (processed) in their navigation systems so that now drivers in Amsterdam can be steered to the nearest available parking for their destination. Those same data have been used by Glimworm IT that created the ParkShark Amsterdam App. Data create business, that is what the data economy is all about.
Quite interesting in this respect the report: "New Business around Open Data: smart Cities and FIWARE". If you are interested in this area the report is a must read. If you only want to understand how to promote the use of a city Open Data by third parties read at least the part 2.4.