Every version of Android is different. And people who develop for Android have to choose which versions their application will support. I suspect the more versions supported, the larger the Application will get. On top of that, there are new features being added all the time. For instance, the USB AOA protocol is not available on older versions of Android. So you are out of luck if you go back too far. And an Android can (as of now) not handle more than 1 USB device. But the way the Android software is written there might be a day when multiple devices are support. But, obviously, on newer versions of Android.
So masquerading an Android device as a different version than it really is is probably not a good idea. Not to mention probably a difficult thing to do. So many things depend on the version number. For example, the device might start downloading incomparable versions of applications.
Although I don't see what Jellybean can do that an older version of android can't
This is the feature list for Jellybean 4.1, the first release. If we listed everything up to Jellybean 4.3 then the list would be 3x as long:
Smoother user interface:
Vsync timing across all drawing and animation done by the Android framework, including application rendering, touch events, screen composition and display refresh
Triple buffering in the graphics pipeline
Bi-directional text and other language support
User-installable keyboard maps
Ability to turn off notifications on an app specific basis
Shortcuts and widgets can automatically be re-arranged or re-sized to allow new items to fit on home screens
Bluetooth data transfer for Android Beam
Offline voice dictation
Tablets with smaller screens now use an expanded version of the interface layout and home screen used by phones.
Improved voice search
Improved camera app
Google Wallet (for the Nexus 7)
High-resolution Google+ contact photos
Google Now search application
USB audio (for external sound DACs)
Audio chaining (also known as gapless playback)
Stock Android browser is replaced with the Android mobile version of Google Chrome in devices with Android 4.1 preinstalled
Ability for other launchers to add widgets from the app drawer without requiring root access