I have extensively used Ubuntu in the past as a desktop or a home server. It is not a lightweight OS. With Google Nexus 7 8GB or 16GB storage capacity, how much of the storage capacity will be left for add-on programs or user data. Talking about Ubuntu add-on programs, these are not lightwieght likewise.
I have not tried Ubuntu 13.04 yet but knowing its past release, I don't really know if the user's experience on Android will be similar. This will be a technology to watch.