Well, since the android kernel was forked from the main linux kernel way back when it started (although maybe not so long ago) because google wanted better power manangement as well as driver support for devices on the mobile platforms the fact it runs android can mean a few things - for instance it may mean that it uses some hardware that the main linux kernel doesnt support out of the box - for which the ARM/Android ubuntu initiative tries to fix up a little.

Then theres the issue that if you did replace Android with Ubuntu or some other linux distribution and did not make it dual bootable, which even if you did would require a keyboard for GRUB or LILIO, you'd end up with a slightly ugly user interface - similar issue that causes some usability issues on touch screen devices and desktop linux and so on... So in short - unless you want to replace all the goodness of android that may be your best option