Banksy

all art is the meaningless expression of the ego of the artist. it should be judged by aesthetics, alone, and not any perceived "message".

In general; yes, but all viewers are inherently subjective. Therefore messages are found and either consciously or subconsciously help determine "worth".

if the artist really had something important to say they would write a polemic

Ah, no. Art has the ability to transcend written and aural language. [but, refer to subjectivity via culture - see those lame HSBC ads]. Art allows apologia to write.