Everybody, even the most ardent unionists, accept that Wales is a nation. The natural status of nations is independence; therefore Wales should be independent - what is there to argue about?
The Scottish Nationalist Mike Mackenzie answers this question on Ian Hamilton QC's blog
This business of maintaining the Union may therefore not be an area in which we are really capable of thinking rationally. Rather our attitudes may stem more from custom and belief than from reason. Those seeking to justify the continuance of the Union are therefore at a loss to find reasoned arguments.
An interesting essay, worth reading and worth discussing!