Now that many Apps are moving online, something a bit beyond the Quicken Online, an API perhaps may help. Could not locate such a feature, hence this feedback.
If the Full Quicken Software is hosted online and only the Web API is leased to Accounting Service Providers. The Developers with the Service Providers can develop Adobe AIR Software that run on Thin Clients like these Nettop-and-Servers. These just have the minimum Linux OS to fire up Adobe AIR.
So, on Startup, the Screen just shows the client AIR screen in which entries are made or reports taken, depending on that client's configuration. This way the developers just need to learn using an Accounting Web API, on Adobe AIR. The Quicken Software is always up to date as it is on their servers. The complex process of bookkeeping, need not be studied for designing the clients. Only the Simplicity of the I/O design and the Power of the Remote API provider defines the service results.
In this Method, the Rich Experience and Capabilities of the Software Company, which is its IP; is well protected. Only the API is deployed, not the binaries.