I have a lingering question. For Hybrid Salesforce Apps, why do we need the 'com.salesforce' cordova plugin? We are able to fetch the Salesforce data using the ForceJS library as it is? :)
The Mobile SDK is more than just database functionality. It gives you access to offline storage, camera, advanced gestures, notifications, device contact and calendars, and so on. You'll want to read more in Native, HTML5, or Hybrid.
Can you access the camera with ForceJS? How about device notifications? Can you access the native contacts and calendars? Mind you, if all you need is literally Salesforce access, feel free to use ForceJS. If you want anything more complicated than that, like a fully functional application, you'll need more than just Salesforce connectivity, which is where the Mobile SDK comes in play; there's no need to import a half-dozen libraries when you can just import the Mobile SDK and be done with it.
There's a few things that you get from the Salesforce SDK, over-and-above what you'd get with just using the forcejs. The big things, in my opinion, are the support of the SmartStore (the encrypted local SQLite DB) and the authentication management. I say this as security is hard, and I'd not advise doing this yourself when there's a team already building it, and it's the same as used in SF1.