There is no such thing as a purely aesthetic anything. If a work of art is apparently a-political that is political in and of itself. A space apparently 'outside' politics only serves to reinforce the 'inside' of politics. You can't have right without wrong, or left without right or hot without cold; you can't have one without the other and as such nothing ever escapes it - not even art.
All art is political because attributing artistry to a thing (be it a pile of bricks, a shed that used to be a boat or a piece of stencil graffiti) suggests that this thing is worth the time, effort and (perhaps most importantly) money that it takes to constitute that thing - these are political considerations.
To place the artwork in a timeless, spaceless space outside such base, coarse things as society and politics is to exempt art from the same moral and practical ties as everything else. It is to exempt the exclusivist, elitist pastime of high art from social critique, thereby erasing its exclusivity and elitism and fixing the artwork as a thing important beyond the social meaning that people attribute it.
The debate over what 'is' art and what 'isn't' is therefore a statement of political power. It is an attempt to prefer one form of human expression above others not only in terms of quality but of type, of kind.
The relatively simplistic pleasures of a Banksy piece are then excluded from the art club. Such obviousness has no place within the frankly stultifying orthodoxies of high art society.
I suggest, therefore, that we should celebrate Banksy's ineligibility for the Turner prize. It is not a reflection upon Banksy's artistic credibility or indeed his quality. It merely suggests the roles his work performs and the roles it doesn't.
Banksy lightens up otherwise dull parts of urbanity. He brings a smile to passing city dwellers and tourists. He makes otherwise empty social spaces meaningful. He creates space for humour and social commentary where there was before only pollution and concrete.
The kind of 'intelligent' art celebrated by the Turner prize however attempts to situate itself far outside of space, far outside of politics and far outside of humour, pollution and urban decay. It therefore creates a space wherein middle class urbanites can experience a highly restricted, sanitised aesthetic that erases dirt, politics, social meaning, class, etc. This is rather like the desire for such people to live in walled communities ('because you can't put a price on security'), to never take public transport ('because its just so dirty, yuck, and the people!'), to send their kids to public schools ('because those inner city youths just would just ruin poor little Tarquin'), etc.
Jonathan, in not nominating Banksy you have paid him the highest possible compliment. I will take dirty, obvious and political over sanitised, meaningless and vacuous any day. Frankly, the lack of intelligence shown in high art and your defence of it just goes to show that if you've got nothing to say, say it's art.