The phrase "A real linux OS phone" is pretty frustrating considering Android exists and is supposed to be "real Linux". What are the main reasons Android is not "real Linux"? (Its hard not to be upset with Google for this situation if the reason is what I think it is).
The userspace in Android is very different from the userspace in GNU/Linux. In GNU/Linux, it is very easy to program using your preferred programming language. This isn't the case in Android, everything is very tightly wrapped up around Dalvik. Before you mention the NDK, let me say that it's only a subset of C++ (no exceptions among other things). Ubuntu on the cell phone also opens up the possibility of things like shell scripting, local webservers, etc.
What this really comes down to is programming freedom that you can find in GNU/Linux that isn't found in Android. You will now have a full fledged computer in your pocket instead of a limited device.