Likely if you filled up 16GB there was a lot of "crap" on there that could easily be dumped to make room for 2GB. It may not be so easy when all you get is < 3GB of usable space before you need to start grooming content on the device.
Correct me if I'm wrong, but if that 2GB is not available even temporarily, you are unable to update the device? If that's the case I'm fine with stating 3+2. Nobody wants to have to go deleting/hunting for space just to get an update, that's too much work. Might as well be using Android.