Grading art isn't subjective, it's qualitative. Instead of tallying the number of  right answers, an art teacher assesses the qualities of the work, such as choice of subject, handling of media, and control of the composition. That means, incidentally, that it's harder to grade art than a spelling test, an arithmetic quiz, or a multiple choice history exam. In general, it's hard to grade any assignment where the answers don't fit neatly on a computer card.

Grading art is a way to give students constructive feedback on their learning and practice, and to be reasonably consistent about the standards we ask each student to meet. 

Grading art with a rubric fairly assesses  4 qualities of any work of art and allows for feedback. The subject, techniques, and medium may change with each assignment, but the criteria remains the same. 

Assessing art is about the processes, not the final product. Creating in the studio is project based learning and the multi-step processes facilitate growth in many areas. The learning happens in the doing, therefore assessment begins from the first sketch to the end product.