Yeah, there clearly is. I think your really missing it quite big.
Firstly of course there is 100% security issues accessing API through javascript. You cant just do that and it will work, because you have the authentication token you first need to have to be able to do any of that. So to avoid confusion for anyone reading that - This is needed.
Also anyone reading theBCMan's comments - No it does not mean you can access the API on the front end of your websites as well.
In terms of what I am referring to TheBCMan is passing data out of an app and back in through 3rd party API servers, I am not sure what else needs to be said, sorry if you do not understand what I am referring to there but never mind.
In terms of SOAP, again, I never mentioned about a security issue there, you are getting the wrong end of the stick, and again, we would not have apps that people can purchase and be allowed by BC if that was the case now would it?
It is not officially support however - hence no documentation on those being available in the scope of open platform.
And in terms of the original posters question - Yes it is, but there are limitations in the scope of the current open platform limitation.
You can build Sage app with a BC app, but it wont be automatically passing data to Sage or doing things automatically, no app will be able to offer that. Do you realise that TheBCMan? there has been partner advisory and developer discussions on this for some time. That only comes via the full API and where you specify the API Integration notification data for a site. Similar is not available to apps yet.
The fact it is also still open beta, methods currently open, not intended to be by BC can and may well be closed. And I gave an example of the Web app api access outside of apps - You may be able to do it, but one week an update could role out and that door will be closed. Because it works does not mean it is intended or wanted by the BC team, so understanding and listening to them, especially with apps is very important.